Reducing reference signals when communicating multiple sub-subframes between a base station and a wireless terminal

ABSTRACT

Methods may be provided to operate a network node in a radio access network, RAN. For example, a first sub-subframe of a subframe may be transmitted wherein the first sub-subframe includes reference signals for a wireless terminal. After transmitting the first sub-subframe, a second sub-subframe of the subframe may be transmitted, wherein the second sub-subframe includes downlink data for the wireless terminal, and wherein the second sub-subframe is free of reference signals. Related wireless terminal methods, network nodes, and wireless terminals are also disclosed.

BACKGROUND

The present disclosure relates generally to wireless communications, andmore particularly, to subframe structures for communications betweenbase stations and wireless terminals.

Packet data latency is a performance metric that vendors, operators, andend-users regularly measure (e.g., via speed test applications). Latencymeasurements may be performed in all phases of a radio access networksystem lifetime, for example, when verifying a new software release orsystem component, when deploying a system, and/or when the system is incommercial operation.

Improved latency relative to previous generations of 3^(rd) GenerationPartnership Project (3GPP) Radio Access Technologies (RATs) was aperformance metric that guided the design of Long Term Evolution (LTE).LTE is also now recognized by end-users to be a system that providesfaster access to the Internet and lower data latencies than previousgenerations of mobile radio technologies.

Packet data latency may be important not only for perceivedresponsiveness of the system. Packet data latency may also be aparameter that indirectly influences throughput of the system. HTTP/TCP(Hypertext Transfer Protocol/Transmission Control Protocol) is thedominating application and transport layer protocol suite used on theInternet today. According to HTTP Archive(http://httparchive.org/trends.php) a typical size of HTTP basedtransactions over the Internet may be in the range of a few 10's ofKbyte up to 1 Mbyte. In this size range, the TCP slow start period maybe a significant part of the total transport period of the packetstream. During TCP slow start, the performance may be latency limited.Hence, improved latency may be shown to improve average throughput, forthis type of TCP based data transaction.

Radio resource efficiency may be positively impacted by latencyreductions. Lower packet data latency may increase a number oftransmissions possible within a certain delay bound. Accordingly, higherBLER (Block Error Rate) targets may be used for data transmissionsfreeing up radio resources and/or potentially improving capacity of thesystem.

There are a number of current applications that may be positivelyimpacted by reduced latency in terms of increased perceived quality ofexperience. Examples of such applications may include gaming, real-timeapplications (e.g., VoLTE/OTT VoIP), and/or multi-party videoconferencing.

Going into the future, there may new applications that will be moredelay critical. Examples of such applications may include remotecontrol/driving of vehicles, augmented reality applications (e.g., smartglasses), and/or specific machine communications requiring low latency.

It should also be noted that reduced latency of data transport may alsoindirectly provide faster radio control plane procedures (e.g., callset-up/bearer set-up), due to the faster transport of higher layercontrol signaling.

Accordingly, there continues to exist a need in the art for methodsand/or systems providing further reductions in latency.

SUMMARY

According to some embodiments of inventive concepts, a method may beprovided to operate a network node in a radio access network, RAN. Afirst sub-subframe of a subframe may be transmitted, wherein the firstsub-subframe includes reference signals for a wireless terminal. Aftertransmitting the first sub-subframe, a second sub-subframe of thesubframe may be transmitted, wherein the second sub-subframe includesdownlink data for the wireless terminal, and wherein the secondsub-subframe is free of reference signals.

According to some other embodiments of inventive concepts, a method maybe provided to operate a wireless terminal in communication with a radioaccess network. A first sub-subframe of a subframe may be received froma base station, wherein the first sub-subframe includes referencesignals for the wireless terminal. After receiving the firstsub-subframe, a second sub-subframe of the subframe may be received fromthe base station, wherein the second sub-subframe includes downlink datafor the wireless terminal, and wherein the second sub-subframe is freeof reference signals.

According to still other embodiments of inventive concepts, a networknode of a radio access network (RAN) may include a communicationinterface configured to provide communication with one or more wirelessterminals over a radio interface, and a processor coupled with thecommunication interface. The processor may be configured to transmit afirst sub-subframe of a subframe, wherein the first sub-subframeincludes reference signals for a wireless terminal. The processor mayalso be configured to transmit a second sub-subframe of the subframeafter transmitting the first sub-subframe, wherein the secondsub-subframe includes downlink data for the wireless terminal, andwherein the second sub-subframe is free of reference signals.

According to yet other embodiments of inventive concepts, a network nodeof a radio access network (RAN) may be provided. More particularly, thenetwork node may be adapted to transmit a first sub-subframe of asubframe, wherein the first sub-subframe includes reference signals fora wireless terminal. The network node may also be adapted to transmit asecond sub-subframe of the subframe after transmitting the firstsub-subframe, wherein the second sub-subframe includes downlink data forthe wireless terminal, and wherein the second sub-subframe is free ofreference signals.

According to more embodiments of inventive concepts, a wireless terminalmay include a transceiver configured to provide radio communication witha radio access network over a radio interface, and a processor coupledwith the transceiver. More particularly, the processor may be configuredto receive a first sub-subframe of a subframe from a base station,wherein the first sub-subframe includes reference signals for thewireless terminal. In addition, the processor may be configured toreceive a second sub-subframe of the subframe from the base stationafter receiving the first sub-subframe, wherein the second sub-subframeincludes downlink data for the wireless terminal, and wherein the secondsub-subframe is free of reference signals.

According to additional embodiments of inventive concepts, a wirelessterminal may be adapted to receive a first sub-subframe of a subframefrom a base station, wherein the first sub-subframe includes referencesignals for the wireless terminal. The wireless terminal may also beadapted to receive a second sub-subframe of the subframe from the basestation after receiving the first sub-subframe, wherein the secondsub-subframe includes downlink data for the wireless terminal, andwherein the second sub-subframe is free of reference signals.

By providing reference signals in some sub-subframes for a wirelessterminal but not others, reference signal overhead may be reducedthereby increasing an efficiency of radio resource usage.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this application, illustrate certain non-limiting embodiments ofinventive concepts. In the drawings:

FIG. 1 is a block diagram illustrating base stations of a radio accessnetwork in communication with wireless terminals and a core network nodeaccording to some embodiments of inventive concepts;

FIG. 2 is a block diagram of a base station of FIG. 1 according to someembodiments of inventive concepts;

FIG. 3 is a block diagram of a wireless terminal of FIG. 1 according tosome embodiments of inventive concepts;

FIG. 4 is a block diagram of a core network node according to someembodiments of inventive concepts;

FIG. 5 is a time-frequency diagram illustrating examples of schedulingsub-subframes of a subframe according to some embodiments of inventiveconcepts;

FIGS. 6-10, 11A, and 11B are a time-frequency diagrams illustratingexamples of scheduling control channels and sub-subframes of multiplesubframes according to some embodiments of inventive concepts;

FIG. 12 is a signaling diagram illustrating control signaling timing forscheduling requests according to some embodiments of inventive concepts;

FIGS. 13 and 14 are a flow charts illustrating wireless terminaloperations according to some embodiments of inventive concepts;

FIG. 15A is a time-frequency diagram illustrating examples of schedulingcontrol channels and sub-subframes of a subframe according to someembodiments of inventive concepts;

FIG. 15B is a time-frequency diagram illustrating examples of referencesignal distributions in sub-subframes of FIG. 15A according to someembodiments of inventive concepts;

FIG. 16 is a flow chart illustrating base station operations accordingto some embodiments of inventive concepts; and

FIG. 17 is a flow chart illustrating wireless terminal operationsaccording to some embodiments of inventive concepts.

DETAILED DESCRIPTION

Inventive concepts will now be described more fully hereinafter withreference to the accompanying drawings, in which examples of embodimentsof inventive concepts are shown. Inventive concepts may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of inventive concepts to those skilled in theart. It should also be noted that these embodiments are not mutuallyexclusive. Components/elements from one embodiment may be tacitlyassumed to be present/used in another embodiment.

For purposes of illustration and explanation only, these and otherembodiments of inventive concepts are described herein in the context ofoperating in a RAN (Radio Access Network) that communicates over radiocommunication channels with wireless terminals (also referred to as UEs,user equipments, user equipment nodes, mobile terminals, wirelessdevices, etc.). It will be understood, however, that inventive conceptsare not limited to such embodiments and may be embodied generally in anytype of communication network. As used herein, a legacy or non-legacywireless terminal (also referred to as a UE, user equipment, userequipment node, mobile terminal, wireless device, etc.) can include anydevice that receives data from and/or transmits data to a communicationnetwork, and may include, but is not limited to, a mobile telephone(“cellular” telephone), laptop/portable computer, pocket computer,hand-held computer, an M2M device, IoT (Internet of Things) device,and/or desktop computer.

Note that although terminology from 3GPP (3rd Generation PartnershipProject) LTE (Long Term Evolution) has been used in this disclosure toprovide examples of embodiments of inventive concepts, this should notbe seen as limiting the scope of inventive concepts to only theaforementioned system. Other wireless systems, including WCDMA, WiMax,UMB and GSM, may also benefit from exploiting ideas/concepts coveredwithin this disclosure.

Also, note that terminology such as eNodeB (also referred to as a basestation, eNB, etc.) and UE (also referred to as user equipment, userequipment node, wireless terminal, mobile terminal, wireless device,etc.) should be considering non-limiting.

FIG. 1 is a block diagram illustrating a Radio Access Network (RAN)according to some embodiments of present inventive concepts. As shown,communications between base stations and one or more core network nodes(e.g., Mobility Management Entity (MME) or Service GPRS Support NodeSGSN) may be provided using respective S1 interfaces. Each base stationBS may communicate over a radio interface (including uplinks anddownlinks) with respective wireless terminals UEs in a respective cellor cells supported by the base station. By way of example, base stationBS-1 is shown in communication with wireless terminals UE1, UE2, UE3,and UE4, base station BS-2 is shown in communication with wirelessterminals UE5 and UE6, and base station BS-n is shown in communicationwith wireless terminals UE7 and UE8.

FIG. 2 is a block diagram illustrating elements of a base station BS ofFIG. 1. As shown, a base station BS may include a transceiver circuit201 (also referred to as a transceiver or radio interface or acommunication interface) configured to provide radio communications witha plurality of wireless terminals, a network interface circuit 205 (alsoreferred to as a network interface) configured to provide communicationswith other base stations of the RAN (e.g., over the X2 interface), and aprocessor circuit 203 (also referred to as a processor) coupled to thetransceiver circuit and the network interface circuit, and a memorycircuit 207 (also referred to as memory) coupled to the processorcircuit. The memory circuit 207 may include computer readable programcode that when executed by the processor circuit 203 causes theprocessor circuit to perform operations according to embodimentsdisclosed herein. According to other embodiments, processor circuit 203may be defined to include memory so that a memory circuit is notseparately provided.

FIG. 3 is a block diagram illustrating elements of a wireless terminalUE of FIG. 1. As shown, a wireless terminal UE may include a transceivercircuit 301 (also referred to as a transceiver) including a transmitterand a receiver configured to provide radio communications with a basestation BS, a processor circuit 303 (also referred to as a processor)coupled to the transceiver circuit, and a memory circuit 307 (alsoreferred to as memory) coupled to the processor circuit. The memorycircuit 307 may include computer readable program code that whenexecuted by the processor circuit 303 causes the processor circuit toperform operations according to embodiments disclosed herein. Accordingto other embodiments, processor circuit 303 may be defined to includememory so that a memory circuit is not separately provided.

FIG. 4 is a block diagram illustrating elements of a core network node(e.g., an MME and/or an SGSN) of FIG. 1. As shown, a core network nodemay include a network interface circuit 401 (also referred to as anetwork interface or a communication interface) configured to providecommunications with base stations of the RAN (e.g., over the S1interface), a processor circuit 403 (also referred to as a processor)coupled to the network interface circuit, and a memory circuit 407 (alsoreferred to as memory) coupled to the processor circuit. The memorycircuit 407 may include computer readable program code that whenexecuted by the processor circuit 403 causes the processor circuit toperform operations according to embodiments disclosed herein. Accordingto other embodiments, processor circuit 403 may be defined to includememory so that a memory circuit is not separately provided.

In LTE, downlink PDSCH (Physical Downlink Shared Channel) assignmentsuse resource elements spread over all OFDM symbols in a 1 ms downlinksubframe. According to some embodiments disclosed herein, latency may bereduced by using PDSCH assignments covering a (consecutive) subset ofsymbols within a subframe. Such a subset of symbols may be referred toas a sub-subframe (SSF), and data assignments covering a SSF areillustrated herein as sPDSCH. To maintain backward compatibility and tobe able to frequency multiplex legacy wireless terminal users, theexisting OFDM modulation may be used, and the sub-subframe division maybe done at the OFDM symbol level. As one example, the duration of asubframe may be 1 ms including 14 OFDM symbols, and the duration of aSSF may be seven OFDM symbols (i.e., 0.5 ms, for the case with a normalcyclic prefix).

By assigning sPDSCH resources within a sub-subframe (i.e., with shorterduration as compared to a full subframe), decoding latency may bereduced since the transmission ends earlier and take less time, even forroughly the same processing capability, assuming that the payload sizeis down scaled appropriately. This reduction in latency may further beused to reduce HARQ (Hybrid Automatic Repeat Request) RTT (Round TripTime) since ACK/NACK (Acknowledge/Negative-Acknowledge) feedback can beprovided earlier from a downlink transmission and UE side processingperspective. If the uplink enables timely transmission of ACK/NACKfeedback and the network processing time related to retransmissions canbe scaled with the same factor as the SSF as compared to the 1 mssub-frame, then the HARQ RTT may be reduced with the same factor. For a0.5 ms SSF, for example, the HARQ RTT may become 4 ms (instead of 8 ms).However, embodiments of inventive concepts described herein are notdependent on a reduction of the processing time.

An example of sub-subframe assignments for wireless terminals UE1, UE2,UE3, and UE4 of FIG. 1 over two subframes n and n+1 is illustrated inFIG. 5. It should be noted that other SSF lengths are possible, and thatall SSFs are not required to have the same duration in terms ofnumber(s) of OFDM symbols. In the example of FIG. 5, wireless terminalUE1 is assigned a full subframe (14 symbols less symbols used forPhysical Downlink Control Channel or PDCCH) over a first frequencyresource for downlink transmission in subframe n, and wireless terminalUE1 is assigned four 2 symbol sub-subframes over a second frequencyresource for downlink transmissions in subframe n+1. Wireless terminalUE2 is assigned two 7 symbol sub-subframes (less symbols used for PDCCH)over the second frequency resource for downlink transmission in subframen, and wireless terminal UE2 is assigned two 2 symbol sub-subframes overthe second frequency resource for downlink transmissions in subframen+1. Wireless terminal UE3 is assigned one 7 symbol sub-subframe (lesssymbols used for PDCCH) over a third frequency resource for downlinktransmission in subframe n, and there is no downlink assignment forwireless terminal UE3 for sub-subframe n+1. Wireless terminal UE4 isassigned one 7 symbol sub-subframe over the third frequency resource fordownlink transmission in subframe n, and wireless terminal UE4 isassigned three 4 symbol sub-subframes over the third frequency resourcefor downlink transmissions in subframe n+1. In addition, legacy controlinformation and reference signals (such as legacy DCI PDCCH and CRS) maybe transmitted, and PDSCH/sPDSCH is not mapped to such occupied resourceelements.

Existing physical layer downlink control channels, Physical DownlinkControl Channel (PDCCH) and EPDCCH (Enhanced PDCCH), may be transmittedonce per 1 ms subframe. Furthermore,

-   -   A PDCCH is distributed over the whole carrier bandwidth, but is        time multiplexed with PDSCH over the first 1-4 symbols in the        subframe.    -   An EPDCCH is distributed over the whole 1 ms subframe, but is        frequency multiplexed with PDSCH and multiplexed onto one or        multiple PRB pairs for localized and distributed transmission        respectively.    -   PDCCH has common search space where all UEs may need to detect        common cell specific control information.    -   Depending whether a wireless terminal UE has been configured for        ePDCCH or not, the wireless terminal UE processor 303 searches        UE specific control information from wireless terminal UE search        space of ePDCCH or PDCCH, respectively.    -   The exact DownLink DL data allocation is given in downlink        control information (DCI) format which may have different        options depending on, for example, a configured transmission        mode.    -   The size of the PDCCH region can change dynamically on subframe        basis, with the size of the PDCCH region being signaled on the        PCFICH (Physical Control Format Indicator Channel) in the        beginning of the 1 ms subframe.    -   The frequency domain allocation of the EPDCCH may be        semi-statically configured by means of higher layer signaling.        Current control channels carry control information, referred to        as downlink control information (DCI). When a wireless terminal        UE is configured with a certain transmission mode, the wireless        terminal will (in each subframe when it is not in discontinuous        reception DRX) attempt PDCCH decoding of a finite number of DCI        formats transmitted on the PDCCH (or EPDCCH) for a number of        candidate PDCCH resource allocations (referred to as a search        space). The DCI format has a CRC (Cyclic Redundancy Check) which        is scrambled by a wireless terminal UE identification (such as a        C-RNTI), and when the CRC matches after descrambling, a PDCCH        with a certain DCI format has been detected. There are also        identifications that are shared by multiple terminals, such as        the SI-RNTI which is used for transmission of system        information.

Different DCI formats may be distinguished by different pay load sizes(i.e., number of bits in the DCI format). Hence, if we have multiple DCIformats of different sizes, a need for UE blind decoding may increasesince each size requires a decoding attempt for each candidate PDCCHresource allocation.

There are currently a number of different DCI formats. See, 3GPP TS36.212, V12.3.0 (2014-12) for DL resource assignments including format1,1A, 1B, 1C, 1D, 2, 2A, 2B, 2C and 2D.

-   -   Format 1: single codeword transmission:        -   1 bit to indicate resource allocation type (type 0 or type            1),        -   ┌N_(RB) ^(DL)/P┐ bits to indicate the resource allocation            (type 0 or type 1),        -   3 bits to indicate HARQ process number (4 bits for TDD),        -   3 bits to indicate new data indicator (NDI) and redundancy            version (RV),        -   5 bits to indicate modulation and code scheme (MCS).    -   Format 1A, 1B, 1D:        -   ┌log₂(N_(RB) ^(DL)(N_(RB) ^(DL)+1)/2)┐ bits to indicate the            resource allocation (type 2),        -   3 bits to indicate HARQ process number (4 bits for TDD),        -   3 bits to indicate new data indicator (NDI) and redundancy            version (RV),        -   5 bits to indicate modulation and code scheme (MCS).    -   Format 2,2A, 2B, 2C, 2D: two codeword transmission:        -   ┌N_(RB) ^(DL)/P┐ bits to indicate the resource allocation            (type 0 or type 1),        -   3 bits to indicate HARQ process number (4 bits for TDD),        -   2×3 bits to indicate new data indicator (NDI) and RV,        -   2×5 bits to indicate modulation and code scheme (MCS).

Here, P is the resource block group size which depends on the systembandwidth and ┌N_(RB) ^(DL)┐ is the number of resource blocks in thedownlink.

As shown above, there are three different resource allocation types(Type 0, Type 1, and Type 2). For example, for Type 0, the systembandwidth is divided into resource block groups (RBGs) which consist ofP PRB pairs, where P=1, 2, 3, 4 depending on total bandwidth. Then,there is one bitmap indicating whether something is scheduled in an RBG,and then a bitmap per RBG. The principle is same/similar for Type 1 andType 2, and resources are allocated in frequency always assuming 1 mslength subframe.

The Downlink Control Information (DCI) for a downlink schedulingassignment may thus include information on downlink data resourceallocation in the frequency domain (the resource allocation or frequencyresource), modulation and coding scheme (MCS), and HARQ processinformation. In case of carrier aggregation, information related towhich carrier the PDSCH is transmitted on may be included as well.

There are also DCI formats for UL grants, DCI format 0, and DCI format 4as well as for power control commands and DCI formats 3 and 3A.

If control channels are only transmitted once per 1 ms subframe as shownin FIG. 5 and/or if control channels and DCIs are designed for PDSCHassignments with durations equal to the duration of the whole subframe,it may be difficult to further reduce latency. Moreover, existingcontrol channels (PDCCH and EPDCCH) may not be suitable for efficientsharing of resources through frequency multiplexing between (legacy) UEsusing 1 ms subframes and UEs using shorter sub-subframes within the samesubframe. For EPDCCH, the entire 1 ms EPDCCH may need to be received inorder to get the scheduling indication. It may thus be difficult to useEPDCCH to schedule shorter sub-subframes because the latency benefit maybe lost when the wireless terminal UE waits until the end of thesubframe to decode the control information and thus the downlink data.For PDCCH, the PDCCH could in theory be used and transmitted more often(i.e., more frequently than 1 ms), but because PDCCH is spread over theentire bandwidth, this may be inefficient, resulting in unnecessaryoverhead.

According to some embodiments of inventive concepts, control information(such as PDSCH sub-subframe assignments) may be signaled more frequentlythan once per subframe (e.g., more frequently than once per 1 ms), withreduced control information payloads relative to existing DCI formats,and only when needed.

For example, downlink control information may be partitioned into fastDCI (which can vary between different sub-subframes) and slow DCI (whichmay change, at most, once per subframe). The fast DCI may be conveyed tothe wireless terminal UE using an sPDCCH transmission(s). The wirelessterminal UE may monitor different sPDCCH candidate resources and attemptto decode an sPDCCH transmission intended for itself. If successful, thefast DCI from the sPDCCH (together with the slow DCI) may be used todetermine an sPDSCH DL assignment or (sPUSCH UL grant) for the UE.Different embodiments may cover configuring sPDCCH resources,partitioning between slow and fast DCI, and/or conveying the DCI to theterminal. In LTE, uplink PUSCH scheduling grants may use resourceelements spread over all OFDM symbols in a 1 ms uplink subframe.According to some embodiments disclosed herein, latency may be reducedby using PUSCH grants covering a (consecutive) subset of symbols withina subframe. Such a subset of symbols may be referred to as asub-subframe, and scheduling grants covering a sub-subframe may betransmitted on a physical channel referred to as sPUSCH.

According to some embodiments, use of fast/slow control information(e.g., DCI) may enable scheduling decisions within a subframe, therebyreducing frame alignment delay and/or contributing to reduction of HARQRTT as compared to using PDCCH alone. In addition, dynamic sharing ofresources between (legacy) terminals using 1 ms subframes and shortersub-subframes may be enabled. Moreover, DCI overhead may be reduced ascompared to re-using the PDCCH but transmitting it more often.

The scheduler in the base station BS (also referred to as an eNodeB) mayallocate downlink PDSCH resources to terminals in the cell served by thebase station, and the base station BS decides whether a wirelessterminal is to be given an assignment (e.g., a downlink assignment) with1 ms subframe duration or an assignment with one or multiplesub-subframes with duration(s) shorter than a duration of the subframe.From a wireless terminal perspective, these assignments may changedynamically from subframe to subframe and may allowimprovement/optimization of the end-user experience. For example, a 1 mssubframe may be better from a throughput perspective, whereas a shortsub-subframe(s) may be better from a latency perspective. For thecommonly used TCP (Transmission Control Protocol) protocol, for example,user throughput may typically be latency limited during slow-start andmay later become throughput limited.

According to some embodiments, resources may be dynamically roughlydivided in the frequency domain between legacy PDSCH subframeassignments and sub-subframe sPDSCH assignments once every subframeand/or once every ms. In such cases, one scheduler may schedule legacy 1ms subframes every subframe and/or ms, whereas a sub-subframe scheduler(operating at higher frequency) may schedule sub-subframes within theresources roughly assigned for such transmissions. Downlink assignmentsfor 1 ms subframes may be conveyed using PDCCH whereas assignments forthe sub-subframes may be conveyed using the sPDCCH.

If a latency sensitive packet arrives after the rough division, it maybe possible for the sub-subframe scheduler to override the previousdivision and schedule a sub-subframe in resources previously assigned as1 ms subframe. It may happen that the terminal receiving the legacysubframe assignment may not be able to correctly decode the PDSCH.

The slow control information (e.g., slow DCI) may be changed at mostonce per subframe and/or once per ms and may be common for allsub-subframes in a given subframe. The slow control information (e.g.,DCI) may either be intended for a specific wireless terminals UE orcommon to a group of several wireless terminals UEs. The slow controlinformation (e.g., DCI) may be:

-   -   Semi-statically configured by higher layer RRC or MAC signaling        and hence changed relatively infrequently (e.g., slow control        information may remain static over a plurality of subframes);    -   Dynamically (re-)configured using a (E)PDCCH (or even a sPDCCH)        transmitted in every subframe; or    -   Static.

In the case that the slow control information (e.g., slow DCI) istransmitted on a (E)PDCCH and is intended for several wireless terminalsUEs, the slow control information (e.g., slow DCI) may be scrambledusing a group RNTI (Radio Network Temporary Identifier) common to all ofthe recipients of the group of wireless terminals. A single wirelessterminal UE can belong to more than one group, and multiple group RNTIsmay thus be assigned to a single wireless terminal UE. The fast controlinformation (e.g., fast DCI) may be intended for a specific UE, and thefast control information (e.g., fast DCI) may thus be scrambled using awireless terminal UE specific identification, such as the C-RNTI (CellRadio-Network Temporary Identifier).

A significant payload reduction in the sPDCCH fast control information(e.g., fast DCI) may be achieved with respect to the frequency domainresource allocation. For example—

-   -   According to some embodiments, the fast control information        (e.g., fast DCI) on the sPDCCH may not include any sPDSCH        frequency domain resource allocation information at all. In such        embodiments, the wireless terminal UE may determine frequency        domain resource allocation information from the slow control        information (e.g., slow DCI), which may be provided using higher        layer configuration and/or PDCCH resource allocation.    -   According to some other embodiments, the fast control        information (e.g., fast DCI) on the sPDCCH may include a short        field indicating one out of a few different frequency domain        resource allocations, predefined and/or conveyed by the slow        control information (e.g., slow DCI) including configuration by        higher layers and/or via PDCCH. Another form of payload        reduction may be to use a shorter wireless terminal UE specific        identification for the sPDCCH with fewer bits than the 16 bits        used for the C-RNTI.

According to some embodiments, further payload reduction may be achievedby indicating more parameters common to all assigned sub-subframes, suchas, MCS (Modulation and Coding Scheme) and MIMO (Multiple Input MultipleOutput) related precoding information. This may be useful when allsub-subframes are assigned to a single wireless terminals UE. Note thatthe control information (e.g., DCI) can carry both DL schedulingassignments as well as UL grants. Even though the present disclosure hasa DL focus, the UL assignments may be covered as well in someembodiments.

According to some embodiments, there may be two ways to configure thesPDCCH resource allocations: Semi-statically configured by higherlayers; and/or dynamically varying from subframe to subframe.

According to some embodiments, information regarding configuration ofsPDCCH resource allocations may be conveyed in the control information(e.g., DCI) of a PDCCH. Such a PDCCH could either be intended for asingle wireless terminal UE (CRC scrambling with C-RNTI), or to a groupof wireless terminals UEs (and have CRC scrambling with an RNTI that ismonitored by several wireless terminals). According to some otherembodiments, a signal similar to PCFICH may be defined that once everysubframe would indicate sPDCCH resources, for example, selecting one outseveral allocations, each allocation being configured by higher layersignaling. In both of these embodiments, the starting symbol of thefirst position in the time domain could also be given as for EPDCCH anddepend on the length of the PDCCH region.

For embodiments with dynamic variations, slow control information (e.g.,DCI) once every subframe may configure the sPDCCH candidate resource(s)in both time and frequency. The physical channel could be either a(E)PDCCH or an sPDCCH.

For each of these embodiments, the sPDCCH may use any number of OFDMsymbols, and may be multiplexed in time or frequency with (s)PDSCH. InFIG. 6, the sPDCCH is transmitted with a contiguous allocation at theband edge in the frequency domain, but it can also be (arbitrarily)distributed in the frequency domain with non-contiguous allocations,similar to EPDCCH. In FIGS. 6-10, the sPDCCH is shown being transmittedonly in the first symbol of each respective sub-subframe, but the sPDCCHmight also be transmitted in multiple symbols of a respectivesub-subframe.

The wireless terminal UE monitors sPDCCH resources and attemptsdecoding, for example, using the relevant (UE specific) RNTI for CRCdescrambling. If the base station BS (eNodeB) has transmitted controlinformation (e.g., DCI) on an sPDCCH for a particular wireless terminalUE, the wireless terminal UE may detect the control information throughsuccessful decoding (including descrambling based on the wirelessterminal specific identification, e.g., RNTI). If the base station BS(eNodeB) is using the resources for PDSCH transmissions (e.g., forlegacy 1 ms PDSCH assignments), decoding may with sufficiently highprobability fail and the terminal will detect that there was no controlinformation (e.g., DCI) on a sPDCCH transmitted to it.

From a wireless terminal UE perspective according to some embodiments,each wireless terminal UE is assigned a group identification (e.g.,RNTI) that is shared with a group of wireless terminals and anindividual identification (e.g., C-RNTI) that is specifically assignedto that wireless terminal. The wireless terminal UE monitors the PDCCHtransmission and attempts to unscramble downlink control informationusing the assigned group RNTI. If a match is found, the corresponding(slow) control information (e.g., slow DCI) may determine the frequencyresource(s) used for any sPDSCH transmissions (for the group of wirelessterminals) in the subframe. Similarly, the UE may monitor the possiblesPDCCH candidate resources and try to unscramble them using itsindividual identification (e.g., C-RNTI). If a match is found, the fastcontrol information (e.g., fast DCI) from the sPDCCH together with thefrequency allocation from the slow control information (e.g., slow DCI)in the PDCCH determines the resources used for downlink datatransmission over sPDSCH, as well as HARQ information and MCSinformation

FIGS. 6-10 illustrate different embodiments of allocating frequency/timeresources for PDCCH, sPDCCH, and/or sPDSCH.

According to embodiments illustrated in FIG. 6, frequency and/or timeresources used by a fast control channel (e.g., sPDCCH) for transmissionof fast control information (e.g., fast DCI), such as, wireless terminalUE assignments of sPDSCH sub-subframes and time resources thereof, maybe configured using higher layer signaling from the base station (e.g.,MAC and/or RCC signaling when the wireless terminal attaches to the basestation). Accordingly, frequency and/or time resources used by sPDCCHmay remain relatively static over a plurality of subframes. Frequencyresources used by the sPDSCH downlink sub-subframes may be consideredslow control information (e.g., slow DCI) and may be signaled once perms using PDCCH to a group of UEs (sharing a same RNTI). While timeresources for sPDCCH may be configured using higher layer signalingaccording to some embodiments, according to other embodiments timeresources for sPDCCH may be configured each subframe using a slowcontrol information transmitted via PDCCH.

In FIG. 6, wireless terminals UE1, UE2, and UE3 may belong to a samegroup sharing a group identification (e.g., a group RNTI), and eachwireless terminal UE1, UE2, and UE3 may have an individualidentification (e.g., an individual C-RNTI). At the beginning of the 1stsubframe, slow control information (e.g., slow DCI) may be scrambledwith the group identification and transmitted over the slow controlchannel (e.g., PDCCH). More particularly, the slow control informationmay include a frequency resource (e.g., the 1 ^(st) frequency resource)allocated for sPDSCH sub-subframes used for transmissions to wirelessterminals UE1, UE2, and UE3 during the first subframe. Accordingly, thefrequency resource used for sPDSCH sub-subframes assigned to thesewireless terminals may not change during a subframe. The group ofwireless terminals sharing the group identification can thus unscramblethe slow control information (e.g., the frequency resource) for thesubframe using the group identification.

As noted above, the time/frequency resources for wireless terminals UE1,UE2, and UE3 to receive fast control information using a fast controlchannel (e.g., sPDCCH) may be configured by higher layer signaling. Atthe time for each transmission of fast control information using a fastcontrol channel (e.g., sPDCCH), each wireless terminal UE1, UE2, and UE3of the group may thus attempt to descramble the fast control channel(e.g., sPDCCH) using the respective individual identification (e.g.,C-RNTI). The fast control information may define a time resource for asub-subframe assigned to the particular wireless terminal. The fastcontrol channel may also include MCS (modulation and coding scheme)information, MIMO (multiple input multiple output) precodinginformation, HARQ ACK/NACK information, etc. for the assignedsub-subframe. According to additional embodiments of FIG. 6, the slowcontrol information may include a plurality of frequency resourcesavailable for sub-subframe assignments, and the fast control informationfor each sub-subframe may include an identification of one of theavailable frequency resources.

In the example of FIG. 6, the fast control channel sPDCCH-1 may be usedto transmit fast control information scrambled with the individualidentification for wireless terminal UE1 with the fast controlinformation defining a time resource for the first sub-subframe sPDSCH-1assigned to wireless terminal UE1. Wireless terminal UE1 may thusdescramble the fast control information using its individualidentification, and receive downlink data over the assigned sub-subframesPDSCH-1 (defined by a frequency resource received via PDCCH and a timeresource received via sPDCCH-1). Because wireless terminals UE2 and UE3are unable to descramble the control information scrambled with theindividual identification of wireless terminal UE1, wireless terminalsUE2 and UE3 will not attempt to receive downlink data over sub-subframesPDSCH-1.

Similarly, the fast control channel sPDCCH-2 may be used to transmitfast control information scrambled with the individual identificationfor wireless terminal UE2, with the fast control information defining atime resource for the second sub-subframe sPDSCH-2 assigned to wirelessterminal UE2. Wireless terminal UE2 may thus descramble the fast controlinformation using its individual identification, and then receivedownlink data over the assigned sub-subframe sPDSCH-2 (defined by afrequency resource received via PDCCH and a time resource received viasPDCCH-2). Because wireless terminals UE1 and UE3 are unable todescramble the control information scrambled with the individualidentification of wireless terminal UE2, wireless terminals UE1 and UE3will not attempt to receive downlink data over sub-subframe sPDSCH-2.

The fast control channel sPDCCH-3 may be used to transmit fast controlinformation scrambled with the individual identification for wirelessterminal UE3, with the fast control information defining a time resourcefor the third sub-subframe sPDSCH-3 assigned to wireless terminal UE3.Wireless terminal UE3 may thus descramble the fast control informationusing its individual identification, and then receive downlink data overthe assigned sub-subframe sPDSCH-3 (defined by a frequency resourcereceived via PDCCH and a time resource received via sPDCCH-3). Becausewireless terminals UE1 and UE2 are unable to descramble the controlinformation scrambled with the individual identification of wirelessterminal UE3, wireless terminals UE1 and UE2 will not attempt to receivedownlink data over sub-subframe sPDSCH-3.

At the beginning of the 2 ^(nd) subframe, slow control information(e.g., slow DCI) may be scrambled with the group identification andtransmitted over the slow control channel (e.g., PDCCH-2). Moreparticularly, the slow control information may include a frequencyresource (e.g., the 2 ^(nd) frequency resource) allocated for sPDSCHsub-subframes used for transmissions to wireless terminals UE1, UE2, andUE3 during the second subframe. Different frequency resources may thusbe allocated during different subframes as shown in FIG. 6.

The fast control channel sPDCCH-4 may be used to transmit fast controlinformation scrambled with the individual identification for wirelessterminal UE1 with the fast control information defining a time resourcefor the sub-subframe sPDSCH-4 assigned to wireless terminal UE1 in thesecond subframe. Wireless terminal UE1 may thus descramble the fastcontrol information using its individual identification, and thenreceive downlink data over the assigned sub-subframe sPDSCH-4 (definedby a frequency resource received via PDCCH-1 and a time resourcereceived via sPDCCH-4). Because wireless terminals UE2 and UE3 areunable to descramble the control information scrambled with theindividual identification of wireless terminal UE1, wireless terminalsUE2 and UE3 will not attempt to receive downlink data over sub-subframesPDSCH-4.

As further shown in FIG. 6, numbers and relative locations (in frequencyand time) of fast control channel assignments for the first subframe(sPDCCH-1, sPDCCH-2, and sPDCCH-3) and the second subframe (sPDCCH-4,sPDCCH-5, and sPDCCH-6) may be the same, but not all such assignmentsare required to be used. As shown in the second subframe of FIG. 6, forexample, a full duration of the second subframe is assigned by fastcontrol channel sPDCCH-4 for sub-subframe sPDSCH-4. Accordingly, andfast control channel assignments sPDCCH-5 and sPDCCH-6 may thus beunused with respect to wireless terminals sharing the groupidentification discussed above. Moreover, a frequency resource for thegroup of wireless terminals UE1, UE2, and UE3 may be unused for some orall of a subframe. For example, sub-subframe sPDSCH-4 may occupy only afirst third of the second subframe (after completion of slow controlchannel PDCCH-2) with a remainder of the second frequency resource beingunused in the second subframe.

According to embodiments of FIG. 7, time and frequency resources used byfast control channels sPDCCH for a group of wireless terminals sharing agroup identification and frequency resources used by sub-subframessPDSCH for the group of wireless terminals sharing the groupidentification may be configured at the wireless terminals using higherlayer signaling from the base station. In such embodiments, frequencyresources used by sub-subframes sPDSCH for the group of wirelessterminals may remain relatively static over a plurality of subframes,and the frequency and/or time resources used by fast control channelssPDCCH may remain relatively static from one subframe to the next.According to some other embodiments, a plurality of frequency resourcesfor sub-subframes sPDSCH may be configured at the wireless terminalusing higher layer signaling, and fast control information for aparticular sub-subframe sPDSCH may identify one of the plurality offrequency resources for that sub-subframe sPDSCH.

Otherwise, base station and wireless terminal operations relating toFIG. 7 may be similar to those discussed above with respect to FIG. 6.In general, a fast control channel sPDCCH may be used to transmit fastcontrol information scrambled with an individual identification for arespective wireless terminal with the fast control information defininga time resource for a sub-subframe sPDSCH assigned to the wirelessterminal.

According to embodiments of FIG. 8, the frequency resources used by fastcontrol channels sPDCCH and sub-subframes sPDSCH may be transmitted asslow control information and signaled once per ms (e.g., once persubframe) using slow control channel PDCCH to a group of wirelessterminals sharing a group identification. As discussed above withrespect to FIG. 6, frequency resources used for sub-subframes sPDSCH maythus change from one subframe to the next. In addition, frequencyresources used for fast control channels sPDCCH may change from onesubframe to the next. Otherwise, wireless terminals and base stationsoperations relating to FIG. 8 may be similar to those discussed abovewith respect to FIGS. 6 and/or 7.

According to embodiments of FIG. 9, a common frequency resource may beused by fast control channels sPDCCH and sub-subframes sPDSCH, and thiscommon frequency resource may be configured at the wireless terminalsusing higher layer signaling from the base station. As shown in FIG. 9,the frequency resource may thus remain relatively static from onesubframe to the next. As before, wireless terminals UE1, UE2, UE3, andUE4 may be assigned a same group identification (e.g., a group RNTI),but different individual identifications (e.g., individual C-RNTI's).

In embodiments of FIG. 9, all wireless terminals of the group maymonitor the slow control channel PDCCH and each fast control channelsPDCCH of each subframe using their respective individualidentifications to determine if a sub-subframe is being assigned. Forexample, a time resource for a first sub-subframe sPDSCH-1 may betransmitted as fast control information using slow control channelPDCCH-1 and scrambled using the individual identification for wirelessterminal UE1. Wireless terminal UE 1 may thus receive this fast controlinformation, and responsive thereto, wireless terminal UE1 can proceedto receive downlink data in sub-subframe sPDSCH-1.

A time resource for a second sub-subframe sPDSCH-2 may be transmitted asfast control information using fast control channel sPDCCH-2 andscrambled using the individual identification for wireless terminal UE2.Wireless terminal UE2 may thus receive this fast control information,and responsive thereto, wireless terminal UE2 can proceed to receivedownlink data in sub-subframe sPDSCH-2.

A time resource for a third sub-subframe sPDSCH-3 may be transmitted asa fast control information using fast control channel PDCCH-3 andscrambled using the individual identification for wireless terminal UE3.Wireless terminal UE3 may thus receive this fast control information,and responsive thereto, wireless terminal UE3 can proceed to receivedownlink data in sub-subframe sPDSCH-3.

A time resource for a fourth sub-subframe sPDSCH-4 may be transmitted asa fast control information using slow control channel PDCCH-2 andscrambled using the individual identification for wireless terminal UE2.Wireless terminal UE 2 may thus receive this control information, andresponsive thereto, wireless terminal UE2 can proceed to receivedownlink data in sub-subframe sPDSCH-4.

A time resource for a fifth sub-subframe sPDSCH-5 may be transmitted asa fast control information using fast control channel sPDCCH-5 andscrambled using the individual identification for wireless terminal UE3.Wireless terminal UE3 may thus receive this fast control information,and responsive thereto, wireless terminal UE3 can proceed to receivedownlink data in sub-subframe sPDSCH-5.

A time resource for a sixth sub-subframe sPDSCH-6 may be transmitted asa fast control information using fast control channel PDCCH-6 andscrambled using the individual identification for wireless terminal UE4.Wireless terminal UE4 may thus receive this fast control information,and responsive thereto, wireless terminal UE4 can proceed to receivedownlink data in sub-subframe sPDSCH-6.

According to embodiments of FIG. 10, a common frequency resource used byfast control channels sPDCCH and sub-subframes sPDSCH may be provided asslow control information and signaled once per ms (e.g., once persubframe) using the slow control channel PDCCH to a group of wirelessterminals UE1, UE2, UE3, and UE4 sharing a group identification. Asfurther shown in FIG. 10, time resources used by fast control channelssPDCCH may also be provided as slow control information and signaledonce per ms (e.g., once per subframe) using slow control channel PDCCHto the group of wireless terminals. Accordingly, frequency resources forsub-subframes sPDSCH and fast control channels sPDCCH may change fromone subframe to the next, and numbers/timings of fast control channelssPDCCH may change from one subframe to the next.

According to embodiments discussed above, a group of wireless terminalsUEs may be provided with information regarding time and frequencyresources for fast control information transmitted via fast controlchannels sPDCCH, once per subframe via PDCCH, or via higher layersignaling from the base station, and time resources for sub-subframessPDSCH assigned to particular wireless terminals of the group may bereceived as fast control information via fast control channels sPDCCH. Awireless terminal may thus combine partial control information receivedvia a fast control channel sPDCCH with less frequently signaled controlinformation in a subframe structure to receive sub-subframe assignments.

While not shown in FIGS. 6-10, a fast control channel sPDCCH may be usedto assign multiple sub-subframes to the same wireless terminal. As shownfor example in FIG. 11A, corresponding to FIG. 6, fast control channelsPDCCH-1 may be used to transmit fast control information assigning twoconsecutive sub-subframes for downlink transmission of data to wirelessterminal UE1 in the first subframe, and fast control channel sPDCCH-4may be used to transmit fast control information assigning threeconsecutive sub-subframes for downlink transmission of data to wirelessterminal UE1 in the second subframe. Otherwise, FIG. 11A is the same asFIG. 6, and the same/similar concepts may apply with respect toembodiments of FIGS. 7 and 8. Similarly in FIG. 11B, corresponding toFIG. 9, fast control channel sPDCCH-2 may be used to transmit fastcontrol information assigning two consecutive sub-subframes for downlinktransmission of data to wireless terminal UE1. Otherwise, FIG. 11B isthe same as FIG. 9, and the same/similar concepts may apply with respectto embodiments of FIG. 10.

In embodiments illustrated in FIGS. 6, 7, 8, 9, 10, 11A, and 11B, sPDCCHis shown with contiguous frequency resource allocations, but accordingto other embodiments, the frequency resource allocations of sPDCCH inFIGS. 6, 7, 8, 9, 10, 11A, and 11B may be distributed in the frequencydomain with non-contiguous allocations. Similarly, in embodimentsillustrated in FIGS. 6, 7, 8, 9, 10, 11A, and 11B, sPDSCH is shown withcontiguous frequency resource allocations, but according to otherembodiments, the frequency resource allocations of sPDSCH in FIGS. 6, 7,8, 9, 10, 11A, and 11B may be distributed in the frequency domain withnon-contiguous allocations.

LTE (Long Term Evolution) is a radio access technology based on radioaccess network control and scheduling. These facts may impact latencyperformance since a transmission of data may need a round trip of lowerlayer control signaling.

FIG. 12 is a signaling diagram illustrating control signaling timing forscheduling requests. In FIG. 12, the data may be created at the wirelessterminal UE by higher layers at time TO, then the wireless terminal UEmodem may send a scheduling request (SR) to the base station eNB, thebase station eNB may process this SR and respond with a grant, so thatthe data transfer can start at T6 in FIG. 12.

Accordingly, one area to address regarding packet latency reductions isthe reduction of transport time of data and control signaling (byaddressing the length of a Transmission Time Interval TTI) and thereduction of processing time of control signaling (e.g., the time ittakes for a wireless terminal UE to process a grant).

UE receiver processing Since the time needed for turbo decoding maydepend on the code block size, latency can be reduced by reducing thecode block size. Hence, if the code block size (or equivalently thetransport block size) is reduced, the decoding result may be availableearlier (for a given decoding capability in terms of number of paralleldecoders). If instead of transmitting a single large code block oflength 6000 bits once every 1 ms, two consecutives blocks of length 3000bits are transmitted every 0.5 ms, a decoding latency for each block maybe roughly (to a first order) halved, while still sustaining the bitrate at roughly the same complexity. Some performance degradations maybe expected (e.g. due to shorter block length), and a tradeoff may beexpected between latency and receiver performance (but not necessarilysystem or end user performance).

“Sub-subframes”

From the disclosure above regarding wireless terminal UE receiverprocessing, there may be further opportunities to reduce latency forwireless terminal UE receiver processing by having PDSCH assignments notonly covering all OFDM symbols in a 1 ms subframe, but also by havingPDSCH assignments (also referred to as sub-subframes or sPDSCH) withshorter durations covering a lesser number of consecutive OFDM symbols.Durations of such assignments may vary from subframe to subframe asillustrated, for example, in FIG. 5.

In future versions of LTE standards, wireless terminals UEs may havePDSCH assignments that span a subset of OFDM symbols in the time domainof a subframe, rather than spanning all OFDM symbols (except symbolsused by PDCCH and other good signals) of a subframe. Note that FIG. 5does not show existing or future signals such as CRS (Cell SpecificReference Signal), CSI-RS (Channel State Information Reference Signals),and/or EPDCCH (Enhanced Physical Downlink Control Channel), meaning thatall resource elements within the resource assignments may not beavailable for data transmission.

One way in which such resource assignments can be conveyed to thewireless terminal UE is through the use of a new form of PDCCH controlchannel (referred to as an s-PDCCH control channel or sPDCCH controlchannel), that may be transmitted in every sub-subframe as described inthe U.S. provisional application “Defining Sub-Subframe Channels ForData Communication Using Separately Provided Frequency And TimeResources” filed concurrently herewith (Attorney Docket No. P46164_US1).Examples of such sub-subframes are illustrated in FIGS. 5, 6-10, and11A-B.

In the example illustrated in FIG. 5, wireless terminal UE1 has one(legacy) PDSCH resource assignment (e.g., 12 symbols) using a firstfrequency resource in subframe n and four sPDSCH resource assignments insubframe n+1 (with each of the four assignments covering 2 symbols)using a second frequency resource. Similarly, UE 2 receives two sPDSCHassignments in subframe n using the second frequency resource, one inthe first slot (e.g., 5 symbols) and another one in the second slot(e.g., 7 symbols). Wireless terminal UE3 receives one sPDSCH assignment(e.g., 5 symbols) in subframe n using a third frequency resource, and noassignments in subframe n+1. Wireless terminal UE4 receives one sPDSCHassignment (e.g., 7 symbols) in subframe n using the third frequencyresource, and three sPDSCH assignments (e.g., 4 symbols each) insubframe n+1 using the third frequency resource.

According to some embodiments disclosed herein, a subframe may include14 symbols over a 1 ms (millisecond) duration. Moreover, a subframe mayinclude two slots, with each slot including 7 symbols over an 0.5 msduration.

Currently, two kinds of reference signals can be used for channelestimation for data demodulation:

-   -   Cell specific reference signals (CRS or CRS reference signals),        which take up 8 to 24 REs (Resource Elements) in a PRB (Physical        Resource Block). Note that CRS cannot be used to demodulate        non-codebook based precoding for multi-antenna transmission.    -   User specific demodulation reference signals (DMRS or DMRS        reference signals), which take up 12 or 24 REs in a PRB. As DMRS        are user specific, DMRS can be used for non-codebook precoding,        but their position at the last two symbols in each slot may make        them less useful from a latency perspective.

Unlike DMRS, CRS is not a precoded reference signal because CRS is cellspecific and not UE specific. Thus, all wireless terminals UEs mayestimate the non-precoded channel from the same CRS REs (resourceelements). Wireless terminals UEs that are configured with a CRS basedtransmission mode that uses codebook based precoding (e.g., TM4), mayreceive knowledge on the precoding matrix used for the CRS in the DCI(e.g., DCI 2 for TM4). Because Cell Specific Reference Signals CRS arecell specific, the Reference Signals RS are sent all the time,independent of load in the cells. Thus, especially if the network isconfigured for non-shifted CRS positions, the CRS REs of one basestation or cell may be subject to heavy interference from neighbor cellCRS transmissions. There are also proposals to remove the CRSs (e.g., toenable DTX for energy savings and/or to reduce interference in thenetwork). In Release 12, for example, the small cell ON/OFF featureenables ceasing CRS transmission for a carrier that is deactivated forall users.

Applying a similar structure to legacy LTE with one set of DMRS for eachsub-subframe may be unsuitable because significant overhead may berequired in terms of a number of REs used for reference signals comparedto the number of REs used for data transmission. Further, the currentDMRSs are placed at the very end of the slot, which may not be adesirable choice with respect to latency.

According to some embodiments of inventive concepts, a short DMRS(S-DMRS) may be transmitted in a first OFDM symbol(s) in a firstsub-subframe in a sequence of multiple sub-subframes (that may beconsecutive sub-subframes) assigned to a specific wireless terminal UE,without transmitting any reference signals in later ones of the multiplesub-subframes assigned to the wireless terminal (unless the precodingmatrix or the channel changes significantly). Note that thesub-subframes do not need to be consecutive. For example, if wirelessterminal B is scheduled for one OFDM symbol after scheduling wirelessterminal A, and then wireless terminal A is scheduled again,transmission of another SDMRS to terminal A may not be needed. Forexample, when the two assignments to the same wireless terminal arewithin one 1 ms subframe or even within a 0.5 ms slot, another SDMRS maynot be needed.

According to some embodiments of inventive concepts, an SDMRS may beincluded in each sub-subframe. Due to the short time duration of thesuggested sub-subframes (e.g., 1-7 OFDM symbols in length), however,inclusion of an SDMRS in each sub-subframe may lead to overuse ofresources allocated to reference signals.

Compared to CRS based channel estimation and demodulation, proposedS-DMRS reference signals may allow demodulation without knowledge of theprecoder matrix used by the base station.

In each sub-subframe including an SDMRS reference signal, placement ofthe SDMRS in the first OFDM symbol in the sub-subframe may enable thewireless terminal to start estimating the channel or building thechannel estimate/estimator while receiving the rest of the sub-subframe.Use of such SDMRS references signals for channel estimation may allowchannel estimation across subframe boundaries, allowing improved channelestimates.

If certain resources are allocated for data transmission to a specificterminal in a sub-subframe of a subframe, the base station BS processor203 may determine whether there is a need to send a SDMRS referencesignal to the wireless terminal UE. According to some embodiments, basestation processor 203 may determine whether the last transmission to thewireless terminal used the same precoder matrix and took place less thanT1 symbols previously, where T1 is a threshold that can be configured.

If not, then base station processor 203 may send an SDMRS referencesignal to the wireless terminal for the sub-subframe, and otherwise,base station processor 203 may use the sub-subframe resources to insteadsend data. When determining whether to include an SDMRS reference signalfor a sub-subframe, base station processor 203 may also consider, forexample, the CQI (Channel Quality Indicator) reported by the wirelessterminal UE and/or some estimate of what Doppler spread the wirelessterminal UE is exposed to (e.g., by considering channel variations inthe uplink).

According to some embodiments, base station processor 203 may beconfigured so that every N^(th) sub-subframe includes an SDMRS referencesignal, where N can count the number of sub-subframes (with N possiblybeing reset with a certain periodicity such as 0.5 ms or 1 ms) or thenumber of consecutive sub-subframes assigned to the wireless terminalUE. For example, the first sub-subframe in which a wireless terminal UEis scheduled in a slot may always include an SDMRS reference signal.

A new control information field (e.g., a new DCI field) in the PDCCHcontrol channel and/or the SPDCCH control channel may indicate for eachsub-subframe whether or not the sub-subframe includes an SDMRS referencesignal. In addition or in an alternative, the new control informationmay indicate for a future sub-subframe whether or not the futuresub-subframe includes sDMRS reference signals. For example, controlinformation in one PDCCH control channel or in one sPDCCH controlchannel may assign a plurality of sub-subframes of a subframe fordownlink/uplink data communication with a wireless terminal, and thecontrol information may indicate for each of the plurality ofsub-subframes which of the sub-subframes do include sDMRS referencesignals and which of the sub-subframes do not include sDMRS referencesignals.

A new control information field (e.g., a new DCI field) in the PDCCHcontrol channel and/or the SPDCCH control channel may indicate for eachsub-subframe a frequency domain density of the SDMRS reference signals.Stated in other words, the new control information field may identify adistribution of the reference signals across a frequency domain of eachsub-subframe including sDMRS reference signals.

According to some embodiments, an SDMRS reference signal may bescheduled for a wireless terminal in a sub-subframe sPDSCH preceding asub-subframe sPDSCH used for data transmission to the wireless terminal.Base station processor 203 may schedule an SDMRS reference signal for aspecific wireless terminal without scheduling a downlink (DL) dataassignment in the same sub-subframe. This use of a precedingsub-subframe may be useful, for example, where there is a sub-subframewithout a significant amount of data to transmit. In this case, basestation processor 203 can use this sub-subframe for an SDMRS referencesignal, and may use more resources for data transmission in the latersub-subframe(s).

According to some embodiments illustrated in FIG. 13, wireless terminalprocessor 303 may perform blind decoding of a special control channelsPDCCH (as discussed above with respect to FIGS. 6-10, and 11A-B) todetermine whether a sub-subframe has been assigned for datacommunication. Predefined resources for sDMRS reference signals may beprovided for each sub-subframe, and the sDMRS reference signals may ormay not be present for that sub-subframe. If a wireless terminalprocessor 303 is configured to listen for transmissions on a resourcewhere a sub-subframe can start at block 1301, the wireless terminalprocessor 303 attempts to blindly decode the SPDCCH control channelusing its current channel estimate h at block 1303. (Optionally, if thewireless terminal has not been scheduled in the last T2 symbols, whereT2 is a threshold that can be configured, the decoding will probablyfail, and this operation can be skipped). If the wireless terminalprocessor 303 can successfully decode the control information (e.g.,control information such as DCI) sent via the SPDCCH control channel(e.g., determined using a CRC check) at block 1305, the wirelessterminal processor 303 can then determine whether or not it is scheduledto receive data in the respective sub-subframe sPDSCH.

According to embodiments of FIG. 13, if wireless terminal processor 303successfully decodes the control information sent via control channelsPDCCH using the current channel estimate h at block 1305 (i.e., thecontrol channel sPDCCH is scrambled using the individual identification,e.g., C-RNTI, for the wireless terminal, and the CRC check passes), datais scheduled for transmission to the wireless terminal during thecorresponding sub-subframe sPDSCH. Wireless terminal processor 303 maythus determine at block 1307 if the assigned sub-subframe sPDSCHincludes new sDMRS reference signals. If the assigned sub-subframesPDSCH includes new sDMRS reference signals at block 1307, processor 303may generate a new channel estimate h′ at block 1309, replace thecurrent channel estimate with the new channel estimate h′ at block 1311,and decode data from the assigned sub-subframe at block 1315.

If wireless terminal processor 303 does not successfully decode thecontrol information sent via control channel sPDCCH at block 1305 (i.e.,the CRC check fails), the control channel sPDCCH may have been intendedfor another wireless terminal (i.e., the control channel may bescrambled using an individual identification C-RNTI for another wirelessterminal), or the channel estimate used by processor 303 may no longerbe sufficiently accurate due to changes in channel conditions or theprecoding matrix. Accordingly, wireless terminal processor 303 maygenerate a new channel estimate h′ at block 1317 based on sDMRSreference signals assumed to be included in the sub-subframe associatedwith the control channel sPDCCH. At block 1319, processor 303 may againattempt to decode the control channel sPDCCH using the new channelestimate h′. If this decoding is successful at block 1321 (i.e., the CRCcheck now passes), the corresponding sub-subframe sPDSCH is scheduledfor data communication with the wireless terminal, and processor 303will proceed to replace the previous channel estimate h with the newchannel estimate h′ at block 1311, and processor 1315 will decode datafrom the assigned sub-subframe at block 1315.

If the decoding fails using the new channel estimate h′ at block 1321(i.e., the CRC check fails), wireless terminal processor 303 assumesthat it is not scheduled in the current sub-subframe, discards the newchannel estimate h′, and maintains the current channel estimate h. (Thismay enable the base station to schedule a different wireless terminalfor a small number of sub-subframes, and then go back to the oldwireless terminal without the need for a new sDMRS).

Operations of FIG. 13 may be repeated for each control channel sPDCCH ofa subframe for which the wireless terminal is configured. If for a givencontrol channel sPDCCH, the wireless terminal replaces the currentchannel estimate h with a new channel estimate h′ at block 1311, the newchannel estimate h′ becomes the current channel estimate h for the nextiteration of block 1303.

Operations of FIG. 14 are the same as those of FIG. 13 with the additionof decision block 1414. Accordingly, FIG. 14 allows for the possibilitythat an SDMRS reference signal may be scheduled for a wireless terminalin a sub-subframe sPDSCH preceding a sub-subframe sPDSCH used for datatransmission to the wireless terminal. Base station processor 203 mayschedule an SDMRS reference signal for a specific wireless terminalwithout scheduling a downlink (DL) data assignment in the samesub-subframe. Accordingly, operations of blocks 1303, 1305, 1307, 1309,1311, 1317, 1319, and 1321 may be performed for sub-subframes includingdata so that the data is decoded at blocks 1414 and 1315, and forsub-subframes including sDMRS reference signals so that data is notdecoded at block 1414.

According to embodiments of FIGS. 13 and 14, wireless terminal processor303 may recursively average its channel estimate over a subframeboundary as long as the control channel sPDCCH CRC checks, leading tobetter channel estimates. The base station can force the wirelessterminal UE to reset its channel estimate either by changing theprecoding matrix, and thus forcing a CRC fail, or by providing anindicator in the control channel sPDCCH to indicate that sDMRS referencesignals are included in the assigned sub-subframe.

FIG. 15A illustrates frequency and time resource allocations for a groupof wireless terminals including wireless terminals UE1 and UE2 over onesubframe according to embodiments discussed above with respect to FIGS.6-10. Slow control information (e.g., scrambled using a groupidentification such as a group RNTI) may be provided using controlchannel PDCCH, for example, to assign a frequency resource includingsubcarriers SC1 to SC12 to the group of wireless terminals for datacommunication during the subframe, and fast control information(scrambled using an individual identification such as an individualC-RNTI) may be provided using control channels sPDCCH to assignrespective sub-subframes sPDSCH to the wireless terminals. According tothe example of FIG. 15A, fast control information of control channelssPDCCH1, sPDCCH3, sPDCCH4, sPDCCH5, and sPDCCH6 may assign respectivesub-subframes sPDSCH1, sPDSCH3, sPDSCH4, sPDSCH5, and sPDSCH6 for datacommunication with wireless terminal UE1. Fast control information ofcontrol channel sPDCCH2 may assign respective sub-subframe sPDSCH2 fordata communication with wireless terminal 2.

FIG. 15B is a diagram illustrating the six sub-subframes of FIG. 15A anda distribution of sDMRS reference signals (illustrated in solid black).As shown, sDMRS reference signals may be included in sub-subframesPDSCH1 which is the first sub-subframe assigned to wireless terminalUE1, and sDMRS reference signals may be included sub-subframe sPDSCH2which is the first sub-subframe assigned to wireless terminal UE2. Inaddition, sDMRS reference signals may be included in sub-subframesPDSCH5 which is assigned to wireless terminal UE1. Accordingly, fastcontrol information of control channels sPDCCH1, sPDCCH2, and sPDCCH5may include indications that the respective sub-subframes include sDMRSreference signals.

According to embodiments illustrated in FIGS. 15A and 15B, sDMRSreference signals may be omitted from sub-subframes sPDSCH3, sPDSCH4,and sPDSCH6, to increase capacity for data communication. For example,the base station may be configured to provide sDMRS reference signals ina first sub-subframe transmitted to a wireless terminal in a subframe(e.g., in sub-subframe sPDSCH1 for wireless terminal UE1, and insub-subframe sPDSCH2 for wireless terminal UE2). The base station mayalso be configured to provide sDMRS reference signals in a sub-subframeif more than a threshold time (e.g., 0.5 ms) has passed since sDMRSreference symbols were provided for that wireless terminal. In theexample of FIGS. 15A and 15B, the channel estimate based on sDMRSreference signals of sub-subframe sPDSCH1 may be sufficient to decodesub-subframes sPDSCH1, sPDSCH3, and sPDSCH4, but sDMRS reference signalsmay be provided in sub-subframe sPDSCH5 for a new channel estimatebecause more than 0.5 ms have passed since the last sDMRS referencesignals for wireless terminal UE1.

According to some embodiments of inventive concepts, reference signaloverhead may be reduced by not transmitting reference signals in everysub-subframe. Accordingly, a first wireless terminal may reuse channelestimates from an earlier sub-subframe even if there is a gap insub-subframes scheduled for the first wireless terminal (e.g., wirelessterminal UE1 reuses channel estimates from sub-subframe sPDSCH1 forsub-subframe sPDSCH3 in FIGS. 15A-B), thereby allowing scheduling ofother wireless terminals (e.g., wireless terminal UE2 in sub-subframesPDSCH2 in FIGS. 15A-B) without the need to retransmit reference signalsto the first wireless terminal. In addition, blind decoding of fastcontrol information transmitted using control channel sPDCCH may beperformed by a wireless terminal to determine whether sDMRS referencesignals are included in the associated sub-subframe, and this may alsoenable averaging of channel estimates across subframe boundaries.

FIG. 16 is a flow chart illustrating operations of a base station BS(also referred to as a network node) processor 203 according to someembodiments of inventive concepts. For a new subframe at block 1601,processor 203 may transmit control information PDCCH (also referred toas slow control information) through transceiver 201 at block 1603 asshown in FIG. 15A. Control information PDCCH may identify a distributionof reference signals sDMRS across a frequency domain of subsequentsub-subframes sPDSCH1, sPDSCH2, sPDSCH3, sPDSCH4, sPDSCH5, and sPDSCH6;and/or control information PDCCH may include a frequency resourceassignment for a group of sub-subframes (e.g., an assignment ofsubcarriers SC1 to SC12 for sub-subframes sPDSCH1 to sPDSCH6) within thesubframe.

At block 1605, processor 203 may transmit control information sPDCCH1(also referred to as fast control information) through transceiver 201to first wireless terminal UE1, and control information sPDCCH1 mayassign sub-subframe sPDSCH1 to first wireless terminal UE1. At block1607, processor 203 may transmit sub-subframe sPDSCH1 includingreference signals sDMRS through transceiver 201 to wireless terminalUE1. According to some embodiments, sub-subframe sPDSCH1 may includereference signals sDMRS and downlink data for wireless terminal UE1.Moreover, as shown in FIG. 15A, transmission of corresponding controlinformation sPDCCH1 and sub-subframe sPDSCH1 may begin at the same timeand/or may be overlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE1 through transceiver 201 for the downlink data ofsub-subframe sPDSCH1. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Operations ofblocks 1605, 1607, and 1609 may be repeated for each sub-subframe of thefrequency resource assignment in the subframe until the subframe iscomplete at block 1611.

At block 1605, processor 203 may then transmit control informationsPDCCH2 (also referred to as fast control information) throughtransceiver 201 to wireless terminal UE2, and control informationsPDCCH2 may assign sub-subframe sPDSCH2 to wireless terminal UE2. Atblock 1607, processor 203 may transmit sub-subframe sPDSCH2 includingreference signals sDMRS through transceiver 201 to wireless terminalUE2. According to some embodiments, sub-subframe sPDSCH2 may includereference signals sDMRS and downlink data for wireless terminal UE2.Moreover, as shown in FIG. 15A, transmission of corresponding controlinformation sPDCCH1 and sub-subframe sPDSCH1 may begin at the same timeand/or may be overlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE2 through transceiver 201 for the downlink data ofsub-subframe sPDSCH2. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Operations ofblocks 1605, 1607, and 1609 may then be repeated for a next sub-subframe(e.g., sPDSCH3) of the frequency resource assignment at block 1611.

At block 1605, processor 203 may then transmit control informationsPDCCH3 (also referred to as fast control information) throughtransceiver 201 to t wireless terminal UE1, and control informationsPDCCH3 may assign sub-subframe sPDSCH3 to wireless terminal UE1. Atblock 1607, processor 203 may transmit sub-subframe sPDSCH3 (withoutreference signals) through transceiver 201 to wireless terminal UE1.According to some embodiments, sub-subframe sPDSCH3 may include downlinkdata for wireless terminal UE1. Moreover, as shown in FIG. 15A,transmission of corresponding control information sPDCCH3 andsub-subframe sPDSCH3 may begin at the same time and/or may beoverlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE1 through transceiver 201 for the downlink data ofsub-subframe sPDSCH3. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Operations ofblocks 1605, 1607, and 1609 may then be repeated for a next sub-subframe(e.g., sPDSCH4) of the frequency resource assignment at block 1611.

At block 1605, processor 203 may then transmit control informationsPDCCH4 (also referred to as fast control information) throughtransceiver 201 to wireless terminal UE1, and control informationsPDCCH4 may assign sub-subframe sPDSCH4 to wireless terminal UE1. Atblock 1607, processor 203 may transmit sub-subframe sPDSCH4 (withoutreference signals) through transceiver 201 to wireless terminal UE1.According to some embodiments, sub-subframe sPDSCH4 may include downlinkdata for wireless terminal UE1. Moreover, as shown in FIG. 15A,transmission of corresponding control information sPDCCH4 andsub-subframe sPDSCH4 may begin at the same time and/or may beoverlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE1 through transceiver 201 for the downlink data ofsub-subframe sPDSCH4. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Operations ofblocks 1605, 1607, and 1609 may then be repeated for a next sub-subframe(e.g., sPDSCH5) of the frequency resource assignment at block 1611.

At block 1605, processor 203 may then transmit control informationsPDCCH5 (also referred to as fast control information) throughtransceiver 201 to wireless terminal UE1, and control informationsPDCCH5 may assign sub-subframe sPDSCH5 to wireless terminal UE1. Atblock 1607, processor 203 may transmit sub-subframe sPDSCH5 withreference signals through transceiver 201 to wireless terminal UE1.According to some embodiments, sub-subframe sPDSCH5 may includereference signals sDMRS and downlink data for wireless terminal UE1.Moreover, as shown in FIG. 15A, transmission of corresponding controlinformation sPDCCH5 and sub-subframe sPDSCH5 may begin at the same timeand/or may be overlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE1 through transceiver 201 for the downlink data ofsub-subframe sPDSCH5. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Operations ofblocks 1605, 1607, and 1609 may then be repeated for a next sub-subframe(e.g., sPDSCH6) of the frequency resource assignment at block 1611.

At block 1605, processor 203 may then transmit control informationsPDCCH6 (also referred to as fast control information) throughtransceiver 201 to first wireless terminal UE1, and control informationsPDCCH6 may assign sub-subframe sPDSCH6 to first wireless terminal UE1.At block 1607, processor 203 may transmit sub-subframe sPDSCH6 (withoutreference signals) through transceiver 201 to wireless terminal UE1.According to some embodiments, sub-subframe sPDSCH6 may include downlinkdata for wireless terminal UE1. Moreover, as shown in FIG. 15A,transmission of corresponding control information sPDCCH6 andsub-subframe sPDSCH6 may begin at the same time and/or may beoverlapping in time.

At block 1609, processor 203 may receive ACK/NACK feedback from wirelessterminal UE1 through transceiver 201 for the downlink data ofsub-subframe sPDSCH6. While block 1609 is shown before transmitting anext sub-subframe/subframe for ease of illustration, the ACK/NACKfeedback for one sub-subframe may be received after transmitting a nextsub-subframe/subframe according to some embodiments. Because thesubframe of FIG. 15A is now complete at block 1611, processor 203 mayreturn to block 1601 for a next subframe. In a next subframe, differenttime and/or frequency allocations may be provided for one or more groupsof sub-subframes.

FIG. 17 is a flow chart illustrating operations of a wireless terminalUE1 processor 303 according to some embodiments of inventive concepts.For a new subframe at block 1701, processor 303 may receive controlinformation PDCCH (also referred to as slow control information) throughtransceiver 301 at block 1703 as shown in FIG. 15A. Control informationPDCCH may identify a distribution of reference signals sDMRS across afrequency domain of subsequent sub-subframes sPDSCH1, sPDSCH2, sPDSCH3,sPDSCH4, sPDSCH5, and sPDSCH6; and/or control information PDCCH mayinclude a frequency resource assignment for a group of sub-subframes(e.g., an assignment of subcarriers SC1 to SC12 for sub-subframessPDSCH1 to sPDSCH6) within the subframe.

At block 1705, processor 303 may receive control information sPDCCH1(also referred to as fast control information) through transceiver 301from base station BS, and control information sPDCCH1 may assignsub-subframe sPDSCH1 to first wireless terminal UE1. At block 1705 a,processor 303 may determine whether sub-subframe sPDSCH1 includesreference signals, for example, based on control information PDCCHand/or sPDCCH1. If sub-subframe sPDSCH1 includes reference signals asshown in FIG. 15B, processor 303 may generate a channel estimate basedon the reference signals sDMRS of sub-subframe sPDSCH1 at block 1706,and processor 303 may receive sub-subframe sPDSCH1 (including downlinkdata) by decoding sub-subframe sPDSCH1 using the channel estimate basedon the reference signals sDMRS of the sub-subframe sPDSCH1. If thesub-subframe does not include reference signals at block 1706 a (e.g.,sub-subframes sPDSCH3, sPDSCH4, and sPDSCH6), processor 303 may receivethe sub-subframe using a previously generated channel estimate from aprevious sub-subframe.

At block 1709, processor 303 may transmit ACK/NACK feedback throughtransceiver 301 for the downlink data of sub-subframe sPDSCH1. Whileblock 1709 is shown before receiving a next sub-subframe/subframe forease of illustration, the ACK/NACK feedback for one sub-subframe may betransmitted after receiving a next sub-subframe/subframe according tosome embodiments. Operations of blocks 1705, 1706 a, 1706 b, 1707, and1709 may be repeated for each sub-subframe of the frequency resourceassignment that is assigned to wireless terminal UE1 until the subframeis complete at block 1711.

Because sub-subframe sPDSCH2 is not assigned to wireless terminal UE1,wireless terminal UE1 may take no action with respect to sub-subframesPDSCH2.

At block 1705, processor 303 may receive control information sPDCCH3(also referred to as fast control information) through transceiver 301from base station BS, and control information sPDCCH3 may assignsub-subframe sPDSCH3 to first wireless terminal UE1. At block 1705 a,processor 303 may determine whether sub-subframe sPDSCH3 includesreference signals, for example, based on control information PDCCHand/or sPDCCH3. Because sub-subframe sPDSCH3 does not include referencesignals as shown in FIG. 15B, processor 303 may receive sub-subframesPDSCH3 at block 1707 using the channel estimate based on the referencesignals sDMRS of sub-subframe sPDSCH1.

At block 1709, processor 303 may transmit ACK/NACK feedback throughtransceiver 301 for the downlink data of sub-subframe sPDSCH3. Whileblock 1709 is shown before receiving a next sub-subframe/subframe forease of illustration, the ACK/NACK feedback for one sub-subframe may betransmitted after receiving a next sub-subframe/subframe according tosome embodiments. Operations of blocks 1705, 1706 a, 1706 b, 1707, and1709 may be repeated for a next sub-subframe sPDSCH4 that is assigned towireless terminal UE1 at block 1711.

At block 1705, processor 303 may receive control information sPDCCH4(also referred to as fast control information) through transceiver 301from base station BS, and control information sPDCCH4 may assignsub-subframe sPDSCH4 to first wireless terminal UE1. At block 1705 a,processor 303 may determine whether sub-subframe sPDSCH4 includesreference signals, for example, based on control information PDCCHand/or sPDCCH4. Because sub-subframe sPDSCH4 does not include referencesignals as shown in FIG. 15B, processor 303 may receive sub-subframesPDSCH4 at block 1707 using the channel estimate based on the referencesignals sDMRS of sub-subframe sPDSCH1.

At block 1709, processor 303 may transmit ACK/NACK feedback throughtransceiver 301 for the downlink data of sub-subframe sPDSCH4. Whileblock 1709 is shown before receiving a next sub-subframe/subframe forease of illustration, the ACK/NACK feedback for one sub-subframe may betransmitted after receiving a next sub-subframe/subframe according tosome embodiments. Operations of blocks 1705, 1706 a, 1706 b, 1707, and1709 may be repeated for a next sub-subframe sPDSCH5 that is assigned towireless terminal UE1 at block 1711.

At block 1705, processor 303 may receive control information sPDCCH5(also referred to as fast control information) through transceiver 301from base station BS, and control information sPDCCH5 may assignsub-subframe sPDSCH5 to first wireless terminal UE1. At block 1705 a,processor 303 may determine whether sub-subframe sPDSCH5 includesreference signals, for example, based on control information PDCCHand/or sPDCCH5. Because sub-subframe sPDSCH5 includes reference signalsas shown in FIG. 15B, processor 303 may generate a channel estimatebased on the reference signals sDMRS of sub-subframe sPDSCH5 at block1706, and processor 303 may receive sub-subframe sPDSCH5 (includingdownlink data) by decoding sub-subframe sPDSCH5 using the channelestimate based on the reference signals sDMRS of the sub-subframesPDSCH5.

At block 1709, processor 303 may transmit ACK/NACK feedback throughtransceiver 301 for the downlink data of sub-subframe sPDSCH5. Whileblock 1709 is shown before receiving a next sub-subframe/subframe forease of illustration, the ACK/NACK feedback for one sub-subframe may betransmitted after receiving a next sub-subframe/subframe according tosome embodiments. Operations of blocks 1705, 1706 a, 1706 b, 1707, and1709 may be repeated for a next sub-subframe sPDSCH6 that is assigned towireless terminal UE1 at block 1711.

At block 1705, processor 303 may receive control information sPDCCH6(also referred to as fast control information) through transceiver 301from base station BS, and control information sPDCCH6 may assignsub-subframe sPDSCH6 to wireless terminal UE1. At block 1705 a,processor 303 may determine whether sub-subframe sPDSCH6 includesreference signals, for example, based on control information PDCCHand/or sPDCCH6. Because sub-subframe sPDSCH6 does not include referencesignals as shown in FIG. 15B, processor 303 may receive sub-subframesPDSCH6 at block 1707 using the channel estimate based on the referencesignals sDMRS of sub-subframe sPDSCH5.

At block 1709, processor 303 may transmit ACK/NACK feedback throughtransceiver 301 for the downlink data of sub-subframe sPDSCH6. Whileblock 1709 is shown before receiving a next sub-subframe/subframe forease of illustration, the ACK/NACK feedback for one sub-subframe may betransmitted after receiving a next sub-subframe/subframe according tosome embodiments. Because the subframe of FIG. 15A is now complete atblock 1711, processor 303 may return to block 1701 for a next subframe.In a next subframe, different time and/or frequency allocations may beprovided for one or more groups of sub-subframes.

ABBREVIATIONS

ACK Acknowledgement

ARQ Automatic Repeat reQuest

C-RNTI Cell Radio Network Temporary Identity

CRC Cyclic Redundancy Check

CP Cyclic Prefix

DL Downlink

DCI Downlink Control Information

DRX Discontinuous Reception

DTX Discontinuous Transmission

HARQ Hybrid Automatic Repeat reQuest

LTE Long Term Evolution

NDI New Data Indication

MCS Modulation and Coding Scheme

OFDM Orthogonal Frequency Division Multiple Access

PCFICH Physical Control Format Indicator Channel

PDCCH Physical Downlink Control Channel

PDSCH Physical Downlink Shared Channel

PHICH Physical HARQ Indication Channel

PUCCH Physical Uplink Control Channel

PUSCH Physical Uplink Shared Channel

PRB Physical Resource Block

RLC Radio Link Control

RRC Radio Resource Control

RAT Radio Access Technology

RA Resource Allocation

RBG Resource Block Group

RNTI Radio Network Temporary Identifier

RTT Round Trip Time

RV Redundancy Version

SC-FDMA Single Carrier-Frequency Division Multiple Access

SSF Sub-SubFrame

TDD Time Domain Division

TDS Time Domain Split

TTI Transmission Time Interval

EXAMPLE EMBODIMENTS Embodiment 1

A method of operating a network node (BS) in a radio access network(RAN), the method comprising: transmitting a first sub-subframe of asubframe, wherein the first sub-subframe includes reference signals fora wireless terminal; and after transmitting the first sub-subframe,transmitting a second sub-subframe of the subframe, wherein the secondsub-subframe includes downlink data for the wireless terminal, andwherein the second sub-subframe is free of reference signals.

Embodiment 2

The method of Embodiment 1 wherein the first sub-subframe includes firstdownlink data for the wireless terminal and wherein the secondsub-subframe includes second downlink data for the wireless terminal.

Embodiment 3

The method of Embodiment 1 wherein the first sub-subframe is free ofdownlink data for the wireless terminal.

Embodiment 4

The method of any of Embodiments 1-3, wherein there is a gap between thefirst and second sub-subframes.

Embodiment 5

The method of any of Embodiments 1-4, wherein the wireless terminal is afirst wireless terminal, the method further comprising: aftertransmitting the first sub-subframe and before transmitting the secondsub-subframe, transmitting a third sub-subframe of the subframe, whereinthe third sub-subframe includes reference signals and/or downlink datafor a second wireless terminal.

Embodiment 6

The method of Embodiment 5 wherein transmitting the first sub-subframecomprises transmitting the first sub-subframe using a frequencyresource, wherein transmitting the second sub-subframe comprisestransmitting the second sub-subframe using the frequency resource, andwherein transmitting the third sub-subframe comprises transmitting thethird sub-subframe using the frequency resource.

Embodiment 7

The method of any of Embodiments 5-6 further comprising: transmittingfirst control information to the first wireless terminal, wherein thefirst control information assigns the first sub-subframe to the firstwireless terminal; after transmitting the first control information,transmitting second control information to the second wireless terminalwherein the second control information assigns the third sub-subframe tothe second wireless terminal; and after transmitting the second controlinformation, transmitting third control information to the firstwireless terminal, wherein the third control information assigns thesecond sub-subframe to the first wireless terminal.

Embodiment 8

The method of Embodiment 7 wherein transmitting the first sub-subframecomprises transmitting the first sub-subframe using a first frequencyresource, wherein transmitting the second sub-subframe comprisestransmitting the second sub-subframe using the first frequency resource,wherein transmitting the third sub-subframe comprises transmitting thethird sub-subframe using the first frequency resource, whereintransmitting the first control information comprises transmitting thefirst control information using a second frequency resource, whereintransmitting the second control information comprises transmitting thesecond control information using the second frequency resource, andwherein transmitting the third control information comprisestransmitting the third control information using the second frequencyresource, and wherein the first and second frequency resources aredifferent.

Embodiment 9

The method of any of Embodiments 7-8 wherein the third sub-subframeincludes reference signals, wherein the first control informationincludes an indication that the first sub-subframe includes thereference signals, wherein the second control information includes anindication that the third sub-subframe includes the reference signals,and wherein the second control information includes an indication thatthe second sub-subframe is free of the reference signals.

Embodiment 10

The method of any of Embodiments 7-9 wherein the first controlinformation is scrambled with an individual identification for the firstwireless terminal, wherein the second control information is scrambledwith an individual identification for the second wireless terminal, andwherein the third control information is scrambled with the individualidentification for the first wireless terminal, and wherein theindividual identifications for the first and second wireless terminalsare different.

Embodiment 11

The method of Embodiment 10 wherein the individual identification forthe first wireless terminal is a C-RNTI for the first wireless terminal,and wherein the individual identification for the second wireless deviceis a C-RNTI for the second wireless device.

Embodiment 12

The method of any of Embodiments 1-6 further comprising: transmittingfirst control information to the first wireless terminal, wherein thefirst control information assigns the first sub-subframe to the firstwireless terminal; and after transmitting the first control information,transmitting second control information to the first wireless terminal,wherein the second control information assigns the second sub-subframeto the first wireless terminal.

Embodiment 13

The method of any of Embodiments 1-6 and 12 wherein transmitting thefirst sub-subframe comprises transmitting the first sub-subframe using afrequency resource, and wherein transmitting the second sub-subframecomprises transmitting the second sub-subframe using the frequencyresource.

Embodiment 14

The method of any of Embodiments 1-6 and 12-13 further comprising: aftertransmitting the second sub-subframe, transmitting a third sub-subframeof the subframe, wherein the third sub-subframe includes referencesignals and downlink data for the wireless terminal.

Embodiment 15

The method of any of Embodiments 1-14 wherein a sum of the durations ofthe first and second sub-subframes is less than a duration of thesubframe.

Embodiment 16

The method of any of Embodiments 1-15 wherein the subframe has aduration of 1 milliseconds, wherein the first sub-subframe has aduration no greater than 0.5 milliseconds, and wherein the secondsub-subframe has a duration no greater than 0.5 milliseconds.

Embodiment 17

The method of any of Embodiments 1-16 wherein the subframe includes nomore than 14 symbols, wherein the first sub-subframe includes no morethan 7 symbols, and wherein the second sub-subframe includes no morethan 7 symbols.

Embodiment 18

The method of any of Embodiments 1-17, wherein the first sub-subframeincludes first downlink data for the wireless terminal and wherein thesecond sub-subframe includes second downlink data for the wirelessterminal, the method further comprising: receiving first ACK/NACKfeedback from the wireless terminal for the first downlink data; andreceiving second ACK/NACK feedback from the wireless terminal for thesecond downlink data, wherein the first ACK/NACK feedback is separatefrom the second ACK/NACK feedback.

Embodiment 19

The method of Embodiment 18, wherein the second ACK/NACK feedback isreceived after receiving the first ACK/NACK feedback.

Embodiment 20

The method of any of Embodiments 1-19 further comprising: transmittingcontrol information to the wireless terminal identifying a distributionof the reference signals across a frequency domain of the firstsub-subframe.

Embodiment 21

The method of any of Embodiments 1-6 and 13-20 further comprising:transmitting control information to the wireless terminal beforecompletion of the first sub-subframe, wherein the control informationincludes an indication that the second sub-subframe is free of thereference signals.

Embodiment 22

The method of Embodiment 21 wherein the control information assigns thefirst sub-subframe to the wireless terminal.

Embodiment 23

The method of any of Embodiments 21-22 wherein the control informationincludes an indication of the first sub-subframe includes the referencesignals.

Embodiment 24

The method of any of Embodiments 21-23 wherein the control informationassigns the second sub-subframe to the wireless terminal.

Embodiment 25

The method of any of Embodiments 1-24 wherein the network node is a basestation.

Embodiment 26

A network node of a radio access network, the network node comprising: acommunication interface configured to provide communication with one ormore wireless terminals over a radio interface; and a processor coupledwith the communication interface, wherein the processor is configured toperform operations of any of embodiments 1-25.

Embodiment 27

A network node of a radio access network, wherein the network node isadapted to perform operations of any of Embodiments 1-26.

Embodiment 28

A method of operating a wireless terminal in communication with a radioaccess network, the method comprising: receiving a first sub-subframe ofa subframe from a base station, wherein the first sub-subframe includesreference signals for the wireless terminal; and after receiving thefirst sub-subframe, receiving a second sub-subframe of the subframe fromthe base station, wherein the second sub-subframe includes downlink datafor the wireless terminal, and wherein the second sub-subframe is freeof reference signals.

Embodiment 29

The method of Embodiment 28 further comprising: generating a channelestimate based on the reference signals of the first sub-subframe,wherein receiving the second sub-subframe comprises decoding the secondsub-subframe using the channel estimate based on the reference signalsof the first sub-subframe.

Embodiment 30

The method of Embodiment 29 wherein the first sub-subframe includesfirst downlink data for the wireless terminal and wherein the secondsub-subframe includes second downlink data for the wireless terminal.

Embodiment 31

The method of Embodiment 30 wherein receiving the first sub-subframecomprises decoding the first downlink data using the channel estimate.

Embodiment 32

The method of any of Embodiments 28-29 wherein the first sub-subframe isfree of downlink data for the wireless terminal.

Embodiment 33

The method of any of Embodiments 28-32, wherein there is a gap betweenthe first and second sub-subframes.

Embodiment 34

The method of any of Embodiments 28-33 wherein receiving the firstsub-subframe comprises receiving the first sub-subframe using afrequency resource, and wherein receiving the second sub-subframecomprises receiving the second sub-subframe using the frequencyresource.

Embodiment 35

The method of any of Embodiments 28-34 further comprising: receivingfirst control information from the base station, wherein the firstcontrol information assigns the first sub-subframe to the wirelessterminal; and after receiving the first control information, receivingsecond control information from the base station, wherein the secondcontrol information assigns the second sub-subframe to the wirelessterminal.

Embodiment 36

The method of Embodiment 35 wherein receiving the first sub-subframecomprises receiving the first sub-subframe using a first frequencyresource, wherein receiving the second sub-subframe comprises receivingthe second sub-subframe using the first frequency resource, whereinreceiving the first control information comprises receiving the firstcontrol information using a second frequency resource, wherein receivingthe second control information comprises receiving the second controlinformation using the second frequency resource, and wherein the firstand second frequency resources are different.

Embodiment 37

The method of any of Embodiments 35-36 wherein the first controlinformation includes an indication that the first sub-subframe includesthe reference signals, and wherein the second control informationincludes an indication that the second sub-subframe is free of thereference signals.

Embodiment 38

The method of any of Embodiments 35-37 wherein the first controlinformation is scrambled with an individual identification for thewireless terminal, and wherein the second control information isscrambled with the individual identification for the wireless terminal.

Embodiment 39

The method of Embodiment 38 wherein the individual identification forthe wireless terminal is a C-RNTI for the wireless terminal.

Embodiment 40

The method of any of Embodiments 28-39 further comprising:

after receiving the second sub-subframe, receiving a third sub-subframeof the subframe from the base station, wherein the third sub-subframeincludes reference signals and downlink data for the wireless terminal.

Embodiment 41

The method of any of Embodiments 28-40 wherein a sum of the durations ofthe first and second sub-subframes is less than a duration of thesubframe.

Embodiment 42

The method of any of Embodiments 28-41 wherein the subframe has aduration of 1 milliseconds, wherein the first sub-subframe has aduration no greater than 0.5 milliseconds, and wherein the secondsub-subframe has a duration no greater than 0.5 milliseconds.

Embodiment 43

The method of any of Embodiments 28-42 wherein the subframe includes nomore than 14 symbols, wherein the first sub-subframe includes no morethan 7 symbols, and wherein the second sub-subframe includes no morethan 7 symbols.

Embodiment 44

The method of any of Embodiments 28-43, wherein the first sub-subframeincludes first downlink data for the wireless terminal and wherein thesecond sub-subframe includes second downlink data for the wirelessterminal, the method further comprising: transmitting first ACK/NACKfeedback to the base station for the first downlink data; andtransmitting second ACK/NACK feedback to the base station for the seconddownlink data, wherein the first ACK/NACK feedback is separate from thesecond ACK/NACK feedback.

Embodiment 45

The method of Embodiment 44, wherein the second ACK/NACK feedback isreceived after receiving the first ACK/NACK feedback.

Embodiment 46

The method of any of Embodiments 28-45 further comprising: receivingcontrol information from the base station identifying a distribution ofthe reference signals across a frequency domain of the firstsub-subframe.

Embodiment 47

The method of any of Embodiments 28-34 and 40-46 further comprising:receiving control information from the base station before completion ofthe first sub-subframe, wherein the control information includes anindication that the second sub-subframe is free of the referencesignals.

Embodiment 48

The method of Embodiment 47 wherein the control information assigns thefirst sub-subframe to the wireless terminal.

Embodiment 49

The method of any of Embodiments 47-48 wherein the control informationincludes an indication of the first sub-subframe includes the referencesignals.

Embodiment 50

The method of any of Embodiments 47-49 wherein the control informationassigns the second sub-subframe to the wireless terminal.

Embodiment 51

A method of operating a wireless terminal in communication with a radioaccess network, the method comprising: responsive to failure decodingcontrol information from the base station using a first channelestimate, generating a second channel estimate based on assumedreference signals in a sub-subframe associated with the controlinformation; and responsive to success decoding the control informationusing the second channel estimate, receiving downlink data of thesub-subframe.

Embodiment 52

The method of Embodiment 51 wherein receiving downlink data of thesub-subframe comprises decoding downlink data of the sub-subframe basedon the second channel estimate.

Embodiment 53

The method of any of Embodiments 51-52, further comprising: responsiveto success decoding second control information using the second channelestimate, receiving downlink data of a second sub-subframe.

Embodiment 54

The method of any of Embodiments 51-52, further comprising: responsiveto failure decoding second control information from the base stationusing the second channel estimate, generating a third channel estimatebased on assumed reference signals in a second sub-subframe associatedwith the second control information; responsive to failure decoding thesecond control information using the third channel estimate, maintainingthe second channel estimate; and responsive to maintaining the secondchannel estimate, attempting to decode third control information usingthe second channel estimate.

Embodiment 55

The method of any of Embodiments 51-54, the method further comprising:receiving initial control information from the base station, wherein theinitial control information assigns an initial sub-subframe to thewireless terminal; and responsive to decoding the initial controlinformation, generating the first channel estimate based on referencesignals included in the initial sub-subframe.

Embodiment 56

The method of any of Embodiments 55 wherein the initial sub-subframeincludes downlink data for the wireless terminal, the method furthercomprising: decoding the downlink data of the initial sub-subframe usingthe first channel estimate; and wherein receiving downlink data of thesub-subframe comprises decoding the downlink data of the sub-subframeusing the second channel estimate.

Embodiment 57

A wireless terminal comprising: a transceiver configured to provideradio communication with a radio access network over a radio interface;and a processor coupled with the transceiver, wherein the processor isconfigured to perform operations of any of embodiments 28-56.

Embodiment 58

A wireless terminal adapted to perform operations of any of Embodiments28-56.

Further Definitions

When an element is referred to as being “connected”, “coupled”,“responsive”, or variants thereof to another element, it can be directlyconnected, coupled, or responsive to the other element or one or moreintervening elements may be present. In contrast, when an element isreferred to as being “directly connected”, “directly coupled”, “directlyresponsive”, or variants thereof to another element, there are nointervening elements present. Like numbers refer to like nodes/elementsthroughout. Furthermore, “coupled”, “connected”, “responsive”, orvariants thereof as used herein may include wirelessly coupled,connected, or responsive. As used herein, the singular forms “a”, “an”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. Well-known functions orconstructions may not be described in detail for brevity and/or clarity.The term “and/or”, abbreviated “/”, includes any and all combinations ofone or more of the associated listed items.

As used herein, the terms “comprise”, “comprising”, “comprises”,“include”, “including”, “includes”, “have”, “has”, “having”, or variantsthereof are open-ended, and include one or more stated features,integers, nodes, steps, components or functions but do not preclude thepresence or addition of one or more other features, integers, nodes,steps, components, functions or groups thereof. Furthermore, as usedherein, the common abbreviation “e.g.”, which derives from the Latinphrase “exempli gratia,” may be used to introduce or specify a generalexample or examples of a previously mentioned item, and is not intendedto be limiting of such item. The common abbreviation “i.e.”, whichderives from the Latin phrase “id est,” may be used to specify aparticular item from a more general recitation.

It will be understood that although the terms first, second, third, etc.may be used herein to describe various elements/operations, theseelements/operations should not be limited by these terms. These termsare only used to distinguish one element/operation from anotherelement/operation. Thus a first element/operation in some embodimentscould be termed a second element/operation in other embodiments withoutdeparting from the teachings of present inventive concepts. Examples ofembodiments of aspects of present inventive concepts explained andillustrated herein include their complimentary counterparts. The samereference numerals or the same reference designators denote the same orsimilar elements throughout the specification.

Example embodiments are described herein with reference to blockdiagrams and/or flowchart illustrations of computer-implemented methods,apparatus (systems and/or devices) and/or computer program products. Itis understood that a block of the block diagrams and/or flowchartillustrations, and combinations of blocks in the block diagrams and/orflowchart illustrations, can be implemented by computer programinstructions that are performed by one or more computer circuits. Thesecomputer program instructions may be provided to a processor circuit(also referred to as a processor) of a general purpose computer circuit,special purpose computer circuit, and/or other programmable dataprocessing circuit to produce a machine, such that the instructions,which execute via the processor of the computer and/or otherprogrammable data processing apparatus, transform and controltransistors, values stored in memory locations, and other hardwarecomponents within such circuitry to implement the functions/actsspecified in the block diagrams and/or flowchart block or blocks, andthereby create means (functionality) and/or structure for implementingthe functions/acts specified in the block diagrams and/or flowchartblock(s).

These computer program instructions may also be stored in a tangiblecomputer-readable medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the functions/acts specified in the block diagrams and/orflowchart block or blocks.

A tangible, non-transitory computer-readable medium may include anelectronic, magnetic, optical, electromagnetic, or semiconductor datastorage system, apparatus, or device. More specific examples of thecomputer-readable medium would include the following: a portablecomputer diskette, a random access memory (RAM) circuit, a read-onlymemory (ROM) circuit, an erasable programmable read-only memory (EPROMor Flash memory) circuit, a portable compact disc read-only memory(CD-ROM), and a portable digital video disc read-only memory(DVD/BlueRay).

The computer program instructions may also be loaded onto a computerand/or other programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer and/or otherprogrammable apparatus to produce a computer-implemented process suchthat the instructions which execute on the computer or otherprogrammable apparatus provide steps for implementing the functions/actsspecified in the block diagrams and/or flowchart block or blocks.Accordingly, embodiments of present inventive concepts may be embodiedin hardware and/or in software (including firmware, resident software,micro-code, etc.) that runs on a processor such as a digital signalprocessor, which may collectively be referred to as “circuitry,” “amodule” or variants thereof.

It should also be noted that in some alternate implementations, thefunctions/acts noted in the blocks may occur out of the order noted inthe flowcharts. For example, two blocks shown in succession may in factbe executed substantially concurrently or the blocks may sometimes beexecuted in the reverse order, depending upon the functionality/actsinvolved. Moreover, the functionality of a given block of the flowchartsand/or block diagrams may be separated into multiple blocks and/or thefunctionality of two or more blocks of the flowcharts and/or blockdiagrams may be at least partially integrated. Finally, other blocks maybe added/inserted between the blocks that are illustrated. Moreover,although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows. Many different embodiments have been disclosed herein, inconnection with the above description and the drawings. It will beunderstood that it would be unduly repetitious and obfuscating toliterally describe and illustrate every combination and subcombinationof these embodiments. Accordingly, the present specification, includingthe drawings, shall be construed to constitute a complete writtendescription of various example combinations and subcombinations ofembodiments and of the manner and process of making and using them, andshall support claims to any such combination or subcombination.

Other network elements, communication devices and/or methods accordingto embodiments of inventive concepts will be or become apparent to onewith skill in the art upon review of the present drawings anddescription. It is intended that all such additional network elements,devices, and/or methods be included within this description, be withinthe scope of the present inventive concepts. Moreover, it is intendedthat all embodiments disclosed herein can be implemented separately orcombined in any way and/or combination.

1. A method of operating a network node (B in a radio access network,RAN, the method comprising: transmitting a first sub-subframe of asubframe, wherein the first sub-subframe includes reference signals fora wireless terminal; and after transmitting the first sub-subframe,transmitting a second sub-subframe of the subframe, wherein the secondsub-subframe includes downlink data for the wireless terminal, andwherein the second sub-subframe is free of reference signals.
 2. Themethod of claim 1 wherein the first sub-subframe includes first downlinkdata for the wireless terminal and wherein the second sub-subframeincludes second downlink data for the wireless terminal.
 3. The methodof claim 1 wherein the first sub-subframe is free of downlink data thewireless terminal.
 4. The method of claim 1, wherein there is a gapbetween the first and second sub-subframes.
 5. The method of claim 1,wherein the wireless terminal is a first wireless terminal, the methodfurther comprising: after transmitting the first sub-subframe and beforetransmitting the second sub-subframe, transmitting a third sub-subframeof the subframe, wherein the third sub-subframe includes referencesignals and/or downlink data for a second wireless terminal.
 6. Themethod of claim 5 wherein transmitting the first sub-subframe comprisestransmitting the first sub-subframe using a frequency resource, whereintransmitting the second sub-subframe comprises transmitting the secondsub-subframe using the frequency resource, and wherein transmitting thethird sub-subframe comprises transmitting the third sub-subframe usingthe frequency resource.
 7. The method of claim 5 further comprising:transmitting first control information to the first wireless terminal,wherein the first control information assigns the first sub-subframe tothe first wireless terminal; after transmitting the first controlinformation, transmitting second control information to the secondwireless terminal wherein the second control information assigns thethird sub-subframe to the second wireless terminal; and aftertransmitting the second control information, transmitting third controlinformation to the first wireless terminal, wherein the third controlinformation assigns the second sub-subframe to the first wirelessterminal.
 8. The method of claim 1 further comprising: aftertransmitting the second sub-subframe, transmitting a third sub-subframeof the subframe, wherein the third sub-subframe includes referencesignals and downlink data for the wireless terminal.
 9. The method ofclaim 1 wherein a sum of the durations of the first and secondsub-subframes is less than a duration of the subframe.
 10. The method ofclaim 1, wherein the first sub-subframe includes first downlink data forthe wireless terminal and wherein the second sub-subframe includessecond downlink data for the wireless terminal the method furthercomprising: receiving first ACK/NACK feedback from the wireless terminalEURO for the first downlink data; and receiving second ACK/NACK feedbackfrom the wireless terminal for the second downlink data, wherein thefirst ACK/NACK feedback is separate from the second ACK/NACK feedback.11. The method of claim 1 further comprising: transmitting controlinformation to the wireless terminal identifying a distribution of thereference signals across a frequency domain of the first sub-subframe.12. The method of claim 1 further comprising: transmitting controlinformation to the wireless terminal before completion of the firstsub-subframe, wherein the control information includes an indicationthat the second sub-subframe is free of reference signals.
 13. A methodof operating a wireless terminal in communication with a radio accessnetwork, the method comprising: receiving a first sub-subframe of asubframe from a base station, wherein the first sub-subframe includesreference signals for the wireless terminal; and after receiving thefirst sub-subframe, receiving a second sub-subframe of the subframe fromthe base station, wherein the second sub-subframe includes downlink datafor the wireless terminal, and wherein the second sub-subframe is freeof reference signals.
 14. The method of claim 13 further comprising:generating a channel estimate based on the reference signals of thefirst sub-subframe, wherein receiving the second sub-subframe comprisesdecoding the second sub-subframe using the channel estimate based on thereference signals of the first sub-subframe.
 15. The method of claim 14wherein the first sub-subframe includes first downlink data for thewireless terminal and wherein the second sub-subframe includes seconddownlink data for the wireless terminal.
 16. The method of claim 15wherein receiving the first sub-subframe comprises decoding the firstdownlink data using the channel estimate.
 17. The method of claim 13wherein the first sub-subframe is free of downlink data for the wirelessterminal.
 18. The method of claim 13, wherein there is a gap between thefirst and second sub-subframes.
 19. The method of claim 13 whereinreceiving the first sub-subframe comprises receiving the firstsub-subframe using a frequency resource, and wherein receiving thesecond sub-subframe comprises receiving the second sub-subframe usingthe frequency resource.
 20. The method of claim 13 further comprising:receiving first control information from the base station, wherein thefirst control information assigns the first sub-subframe to the wirelessterminal; and after receiving the first control information, receivingsecond control information from the base station, wherein the secondcontrol information assigns the second sub-subframe to the wirelessterminal.
 21. The method of claim 13 further comprising: after receivingthe second sub-subframe, receiving a third sub-subframe of the subframefrom the base station, wherein the third sub-subframe includes referencesignals and downlink data for the wireless terminal
 22. The method ofclaim 13 wherein a sum of the durations of the first and secondsub-subframes is less than a duration of the subframe.
 23. The method ofclaim 13, wherein the first sub-subframe includes first downlink datafor the wireless terminal and wherein the second sub-subframe includessecond downlink data for the wireless terminal, the method furthercomprising: transmitting first ACK/NACK feedback to the base station forthe first downlink data; and transmitting second ACK/NACK feedback tothe base station for the second downlink data, wherein the firstACK/NACK feedback is separate from the second ACK/NACK feedback.
 24. Themethod of claim 13 further comprising: receiving control informationfrom the base station identifying a distribution of the referencesignals across a frequency domain of the first sub-subframe.
 25. Themethod of claim 13 further comprising: receiving control informationfrom the base station before completion of the first sub-subframe,wherein the control information includes an indication that the secondsub-subframe is free of the reference signals.
 26. A network node of aradio access network, RAN, the network node comprising: a communicationinterface configured to provide communication with one or more wirelessterminals over a radio interface; and a processor coupled with thecommunication interface, wherein the processor is configured to,transmit a first sub-subframe of a subframe, wherein the firstsub-subframe includes reference signals for a wireless terminal, andtransmit a second sub-subframe of the subframe after transmitting thefirst sub-subframe, wherein the second sub-subframe includes downlinkdata for the wireless terminal, and wherein the second sub-subframe isfree of reference signals.
 27. (canceled)
 28. A wireless terminalcomprising: a transceiver (MI) configured to provide radio communicationwith a radio access network over a radio interface; and a processorcoupled with the transceiver wherein the processor is configured to,receive a first sub-subframe of a subframe from a base station, whereinthe first sub-subframe includes reference signals for the wirelessterminal, and receive a second sub-subframe of the subframe from thebase station after receiving the first sub-subframe, wherein the secondsub-subframe includes downlink data for the wireless terminal, andwherein the second sub-subframe is free of reference signals. 29.(canceled)